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Yes these are the maximum configurations.
Minus 2 seats for every row sold as Club on the short-haul fleet.

Mexico is essentially operating as an amber country now and the route has been busy. I think they'd operate as some people can Q at home and then take test to release after 5 days

According to the BA Source the maximum C/Y mix would be
A321 56C 136Y = 192 (220Y)
A320 48C 108Y = 156 (180Y)
A319 36C 90Y = 126 (144Y)

788 35J/25W/154Y -> 80J/154Y
789 8F/42J/39W/127Y -> 89J/127Y

I think the wide body substitution really only works if there is very high J demand. I am assuming that Wold Traveller Plus and above would be Club Europe and ignoring cargo. I am also ignoring the opportunity costs of being able to redeploy the substituted A32x elsewhere in the short haul network and assuming that the 787 would otherwise be sat idle. None of these are safe assumptions. I think the reality is that BA does have an amount of flexibility, when you look a bit deeper substituting A32x for wide bodies does not seem a great option, while reactivating the A321ceos will cost them money. Its a good job they have a lot of slots, crew and A320s being rotated through short-term storage. My guess is there is still a lot of flexibility compared to summers gone by.

just a couple of very pedantic corrections
319 is a max of 40 CE,not 36, leaving 84 ET seats and
788 is 60 CE seats incl WTP and 154 ET seats

Currently BA are looking at all options depending on government decisions/ traffic light systems and no doubt will operate a schedule which will work for them in terms of revenue and cost, like any airline would do who is trying to catch up with business, ie there are talks already about 777s doing the likes of ATH and other destinations if demand and costs involved make it worth their while.

From speaking with friends in BA Ops, it seems that no one is really clear of which countries will be in the green/amber/red categories and, crucially, if they'll have reciprocity on the other side. The plans are, therefore, highly speculative and mostly revolve around a progressive increase in June. A380s are rumoured to be returning at some point in the summer season.

A couple of things that I've been told with a higher degree of certainty are:
1. Passenger Locator Form to be added (how, don't know) to the e-Gates from next week
2. T3 to reopen with 'leisure' routes during the summer. Which routes, depends on the list (see above)
3. Most airlines will leave T5 beside AA (AA people told me they'll just leave JFK, BA told me they keep all the routes) and QR
4. HAL plans to use T4 to host the 'red' arrivals, although that will take time just to set up the IT and see what hasn't been eaten by rats in the meantime

Bottom line = uncertainty reigns supreme still. And BA is being cautious, which isn't bad.

Point 4 is a very good idea indeed

Makes sense for direct flights but how will they handle indirect flights ? The govts policy from some destinations of banning direct flights (Dubai etc) means that "red" arrivals are coming in from hubs all over the place, mixed in with amber (or possibly even green if say Spain makes the list!)

That part of the regulations never made sense to me, would have made more sense to have people on direct flights where they could have been easily identified and not mingling with low risk passengers

It is. In practice it can be a logistical nightmare; a terminal move is usually quite labour intensive and doing it on a whim, after a tweet from Shapps has given only a few days' advance notice, will generate a considerable amount of hair pulling and swearing.

Gibraltar has just announced it won't require UK visitors to take tests to visit once tourism restarts - I imagine BA will be adding even more GIB rotations shortly!

That'll be an interesting one seeing as high winds can often divert GIB flights to Malaga and into a different set of arrival requirements. Fingers crossed Gibraltar and Spain will have a unified approach in to such matters as arrival requirements.

It was first ‘suggested’ several weeks ago, and HAL has been planning enablement since then. Not quite everything is managed on tweets from ministers, even if it often feels otherwise.

When you move from one terminal for the first time to the other you need to ensure:
1. GSE availability
2. Access for your team (above & below the wing, either contracted and not). Even simple things like ensuring your badges can open doors, rostering etc.
3. Set up of the local BRS to recognise the local airline's specific rules. T4 is for instance different from T3, or T5's.
4. Ensure your IT kit is in place or, if it's CUTE, that it's loaded on the local sets
5. Various miscellanea that will always trip you up, like for instance bussing crews from their report location to the new stand and viceversa.

History suggests (see last year) that a country can move from one tier to another with very little notice, at the drop of a hat or a tweet. HAL might be up and running, but unless they and the airlines have all set up a parallel station for everyone in T4, then things take a little bit of time. The T3 reshuffle in, when was it, 2016? took a while and so did the Gatwick move. Sure, it might be one or two flights and not a larger contingent, but it's still something that requires time. I'm not saying it's impossible or God forbid criticising HMG (learned my lesson here!) but merely pointing it out. Over & out.

Rumours that the Greek islands of Corfu, Crete and Rhodes will be on the green list. If true I guess BA will increase frequencies quickly?

Well, this from gov.uk:

The Foreign, Commonwealth & Development Office (FCDO) advises against all but essential travel to Greece based on the current assessment of COVID-19 risks. The FCDO is not advising against travel to the islands of Rhodes, Kos, Zakynthos, Corfu and Crete.

...does rather drop a hint! Same as the removal of Portugal (ex-Azores) from the 'no essential travel' list...

BRS?