Welcome!
What is most important in a filter is the amount of water it can turn over. This will be in gallons per hour or litres per hour.
You want something with at least 3x the gph (or about 11-12 lph) the size of your tank. So for your 50 gallon tank, you want something that says it's rated at least 150 gph or ~570 lph. But more is always better.
Canister filters are recommended over internal or waterfall filters, because they generally hold much more media.
I think that Hagen makes their products internationally. A good filter for your tank would be the Fluval 404 or 405.
On their UK site, they list the 405 as 220V, 60Hz. Not sure if that would work for you:
http://www.hagen.com/uk/aquatic/product ... 2160030401
This is just a suggestion though. You may be able to find other filters that will work. Another filter you can keep an eye out for is the Rena XP3.
You may also find something at a local pet store, which would be compatible of course. Just be wary of pet store employees. Most know very little about turtles, and are just out to sell you products you don't need or that won't work for turtles.