I'm far from the most experienced spark in here but here's something for comparison and one I gratefully lost today.
3 x 5m house extension in outer London. Requires sockets, down lights, Outside lights and sockets. TV co-ax connection and Part P certificate. Builder is somewhat disorganised so attracts my PITA tax of 10% and I'm not VAT registered. Board has space for extra circuits.
I quoted £1275 and lost it to a quote of £600! Mate - you are welcome to it at that rate. FFS, I can charge that for 3 / 4 days subbing and then nothing is my problem - fit and forget.
Expecting a call in about a month - " just a few things to finish off and we'll need you to supply a certificate"