Bicester Village - yes
Westfield - just go into central London instead
If you do not find anything at Bicester Village; walk down Jermyn Street, you will have options like TM Lewin that are within your budget - if no luck, Marks &...
Even though you say it is an informal wedding, wear a plain tie - if you really wanted to keep it casual, a navy plain knit. As for the pocket square (assuming you wear a tie) plain white cotton in tv-fold.
In the last 6 weeks I have had three shoes full resoled (with toe segs) by Kokos
Crockett & Jones for Kilgour (loafer): also got: repaired inside the heel rear
Cheaney for Gieves & Hawkes (oxford): also got: new laces, new...
October is enough time for most tailors, ideally order the suit by the end of next month (depending on the tailor); this will allow you to wear the suit a few times before the wedding.
If you go mtm, the process will be shorter.