You need to convert in multiples of 10k. The details are hidden away on the website under priorityclub.com then click Redeem then Europe then Airline Miles.

Unless you need the miles for immediate use, you may want to sit on the points as they never expire. I used 20k points to book 2 nights at a Holiday Inn Express in New England for the Columbus Day holiday weekend in October. The usual price of that room? $320 + tax for 2 nights! It was well worth missing out on 5,000 BA Miles for that!

(Of course, if you want to collect Priority Club points, you're better off chucking your Priority Club Visa and getting a standard Amex card. 1 Amex Membership Reward point = 2 PC points. It is surreal that this is a better deal than using PC's own card!)