Could anyone tell me how to hire a band (not too expensive) for an 18th bday party in london? recommendations?

Hello all! =) i have my 18th birthday coming up in march and i really want to do something special! I was thinking of renting a venue such as the top floor of a pub in soho, buy food and provide some drinks for my guests of about 50 people. i dont even know if it is achievable but i want to try!

I also really want to have a live band play on the night to give it an extra bit of oomph =) but i do not know which bands are good to book. i dont even know how to book. whether i am supposed to negotiate a price or not. i am looking for a band to play indie, maybe rock and then dance and urban later on in the night. i have roughly 250 pounds budget for the band. is that too little for what i am asking for? i would really appreciate any help on this!!! thankyou very much xxxx