I haven`t found any ex-demo/open box white Silver 50 7G so the prices you see with dealers are same, but you should call few shops and ask can they offer you better deal! Let`s say Richer Sounds, Hifix, Seven Oak Sound Vision, Peter Tyson and Audio T for starters. At the same time you can ask if they have any open box / ex-display pairs available. I only found white Silver 100 7G for 570£, but they are too large for you.
Few thoughts / questions
- A40 should be mounted below the tv if possible as you have just about right amount of space available!
- For S50 7G where exactly you planned to mount them in the alcoves? The current Bose speakers each side are mounted way too high. The ideal height for the new front main speakers would be close to the A40 level so the tweeters would be roughly at seated ear level and not high up near ceiling! I would also make sure they wouldn´t go too close to tv or the sidewalls horizontally so something between to find good compromise visually aswell.
- Surround speakers can be placed higher on the side walls and aimed down and that seems to be your only option with the mirror taking lot of space at rear wall and pictures/door at side walls, but i would also have them at identical height each side as the Boses aren´t now. I guess something small and preferably from Monitor Audio. Naturally A10 pair would be ideal, but they are fairly deep/bulky with the table stand used as wall bracket. If not issue then keep on eye of used white pairs!
D-line trunking to hide wires if required: