Just to add a bit about the BA lounges - there are two business lounges in T5A (North and South) and a third one in T5B which is usually quieter*, but getting to/from it requires careful planning and some extra time (see the BA forum if interested).

For a first visit to T5, I would check the occupancy of each lounge - there are screens showing this and also accessible by web page at https://ba-uk-occupancy.livereachmedia.com/

*When writing this, I checked and T5B is rammed - the word must have got out!