I'm fairly sure that domestic segments on a CE RFS ticket (I had one booked just last week) don't have the magic 'Business UK' printed on the boarding pass or are booked into a fare bucket that allows lounge access.

So I'd say strictly speaking you're not allowed in.

However, if you have a print out of your itinerary and ask in the lounge if you're allowed in they might be happy to do so. I think it's more a quirk of the system that there's no redemption 'business UK' rather than a deliberate scheme to keep people out.

If they don't give you access, as CWS says they're a good bunch there and so they'll likely help you get back into the main area. If they don't there's a long winded way to get back and that's to exit through baggage reclaim and go through security again.

Not ideal, but it's a last resort.