I'm aware of AdultDVDMonthly in the UK, which should be the first thing to come up if you google "Adult DVD Rental UK". I can't speak for renting from other countries in Europe, but you can rent from AdultDVDMonthly if you're based in the UK.
The only issue might be that I gather that these services don't always deliver quickly (the DVDs are shipped from Mainland Europe), the selection seems to be a bit limited (do they replace the broken ones/ones that haven't been returned by other members?), and all the rental sites seem fairly expensive for what they offer. AdultDVDMonthly are probably the best of the bunch, I guess.
My advice is you're better off buying DVD's from the US, from secondhand sites like AdultDVDMarketplace, or even the International Trading Forum here on AdultDVDTalk. You'll probably get more titles (per month) for what you pay, and the best part is you get to keep them at the end!!
Hope this helped you out somewhat.
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