Just to update, it took maybe 30 minutes to go from T5 to T3 and vice versa. Absolutely no issues. In fact I also paid a brief visit to the new CX lounge after shopping and was granted entry without issue. Shopping was good in T3 and the new CX lounge seems to be the one to beat. The BA T5 galleries lounges are overcrowded and dirty. The advertisement center on the galleries terrace is appalling -sad to see BA lounges fall so far I used to rank them as some of the best.