If you have a portfolio site and testimonials from past clients, £20k isn't that great. I worked with a guy at a small(ish, <100 employees) company who started at £34k with a similar background to that.

Sounds between acceptable (Central London) and good to me. With an 18yo there's the risk that you decide to ship out to uni, travel the world, or are bonkers but no-one's noticed yet.