Polly Hand is a chiropractor in Balham who is also a doula. She's lovely, so it might be worth giving her a call to discuss. I'm sure she'd speak to you about the role of a doula. If you google her I think her details will come up.
Giving birth is obviously quite daunting, but you will be fine with or without a doula! Just try to be as relaxed as you can, breathe very very deeply through the contractions and be prepared to 'go with the flow / listen to the midwives' as it rarely goes to any type of plan.