Invest in a reasonable quality suit and a variety of dress shirts.
If this is your first suit, I'd opt for charcoal grey, which can be paired with either black or brown shoes, and is dark enough for a formal environment. You don't necessarily need to wear the jacket all the time, but it's a piece that will see you pretty much anywhere if you get the fit and quality right.
For a lower budget, Next do 100% wool suits for £140
Suits Collection | Tailoring & Suits | Mens Clothing | Next Official Site - Page 7
For a higher-end suit, look at TM Lewin. Some are reduced to less than £300, like this one:
Heston Slim-Fit 2-Button Grey Suit | T.M.Lewin
Reiss also do some of the best suits on the highstreet, pop in to see if there's any in the sale.
If your really stretched for money, don't neglect TK maxx or outlet stores.