Trickiest thing about improving it is to retain the fact that it is WT+, and not Club-.

From a quick list, possible areas to improve are (a) dedicated check in; (b) lounge access; (c) wider seat/more pitch; (d) better food; (e) noise canceling earphones; (f) power sockets; (g) priority bags.

(a) is out of step with the current trend at LHR for OLCI/SSCI. Would be nice at outstations though.
(b) would be very nice, but I think a step too far, and not something Silver/J pax would welcome. BA lounges are crowded enough.
(c) is a major reworking. New seats for both. Lose a row for pitch only, or lose a column for width only. That would be very nice, but it's a *big* step forward. Fewer seats = price hike.
(d) possibly. Easy enough to do.
(e) small perceived benefit to the pax vs rewiring/seat retrofits. Hmm. Not sure that's cost effective.
(f) ditto (e), perhaps with a higher benefit to pax.
(g) Yeah, right. Works so well already.

I reckon food, headphones and power. Maybe one of the Club wines (ie a straight red/white choice still, but a better choice than in Y).