Ditto - I had been driving a company car last year, was my first car.
I only had £12k to spend and wanted a 330d Sport, so things were pretty limited for me. I went to Car Giant and got lucky. Ended up getting a November 2002 330d Sport with 97k on the clock. FBMWSH at Sytner's BMW, and seems like a very sound car so far (only had it 2 weeks).
You have more money, so I would recommend going to a dealer and getting something with a less mileage which you will get a dealer warrantee with. Car Giant are superb value for money, and the cars are generally very sound. However, they are usually high mileage and won't come with any warrantee.
If you do decide to slash your budget, then Car Giant is the place to go for absolute value for money, and cars with a history.
In terms of insurance, expect to pay a fair bit if you've not got a NCB. I'm 22 and had to cough up £1,600 for the privilage.