Welcome to the forum. Wow, you've left it late to spend that one!
Yes, the antitrust law rubbish means you can't earn or spend BA miles on AA flights between London and the USA and vice versa. The 25% miles earning applies to the cheapest economy fares only - the more expensive economy fares earn full mileage, as do all premium cabins (with bonuses).
Since your MCO is for BA only (I assume that is the restriction) and you want to collect AA miles, I suggest this:
Look at World Traveller Plus fares from Canada (e.g. YVR, YYZ) to the UK. This is BA's paid premium economy cabin (unlike UA's E+) and (as well as a much nicer cabin and seat) offers full miles plus a cabin bonus in AA's program. If you're flexible (i.e. build in a Saturday night stay) you can probably get a return WT+ flight to London for not too much more than the MCO (i.e. pay a small extra yourself). Have a long weekend or longer in the UK as a holiday ^
If you're purely after the most miles from it and want to fly a little, take a look at going to a 'close' European place as part of your journey (e.g. FRA, CDG, AMS) which you may be able to add on to your base journey to the UK on BA for not much more money, again crediting the miles to AA. If you do this as part of the combined WT+ ticket, the Euro legs book in to full fare economy so again get you 100% miles. Note however that this will either work nicely or send the price sky high on ba.com (by giving you a full fare, very expensive WT+ fare instead of the discounted one) which is why I suggest you try it, but don't be too surprised if it's not working.
(Then in addition to the above, factor in getting to Canada on AA as cheaply as you can).
Alternatively, you may like to phone BA and see if they'll extend the MCO to give you more time to look at options like this - worth a try?