I am based in the UK too (hi

) and i buy from then USA all the time and on the website i personally buy from there is never a £ stated too.
I think you need to state a definate price at which your items will sell for not an approxiamte price, so i would just let the international buyers fugure things out for themselves. Could get very messy like you mentioned exchange rates fluctuate so much.
Also i think with paypal if people send you payments not in your own countries currency ie they dont send payment in £'s YOU as the seller will be charged extra for the converstion . Must admit i'm not toally sure but it could be something you could check out.
Have you thought about using another primary payment option like protx? ALot cheaper than Paypal, and some people feel it looks more professional.