Blake, just make whatever bid you want using this e mail address - info@londoncoins.co.uk - giving all your details: name, address, e mail address. Describe the coin and supply the lot No.
That's what I did on my first bid with LCA. I was automatically given a bidder number at that point (well either immediately, or when I'd won the coin).
Or when you've made your bid, call LCA and speak to Semra Cetin. Ask for acknowledgement, and how you get your bidder number.