What do people wear in London?

This is our (my husband & I) first time to go to the UK, and we hope to buy clothes here in our country that will also be wearable in London. What do you (ladies & men) usually wear in the office & casually? We are in our late twenties. I hope you guys can give me some tips on what to buy for our stay in Middlesex, clothes, shoes, and other things you'd advise. Thanks!