You should definitely avoid having it show up on a statement, Mumsnet are all over tbat, they are apparently very keen to explain what it means.
I always buy the PG of a girl I'm seriously considering seeing, it adds very little to the overall cost of a punt, and there have been a few occasions where I've decided against seeing someone after getting a better look at her.
I've used Ukash a few times, £30 gets you 30 credits and 3 freeviews (smaller amounts don't give freeviews). I've also used PAYG phone credit: though this is more expensive (0.5 credits per £), it's handy if you just want a small amount to check out a new girl, and it just means that your next petrol bill is maybe £10 more expensive if you top up at a filling station: completely traceless.