Sounds like a nice idea. Regardless of filtration, the best way of removing solid waste like poop is by regular partial water changes using a
gravel vac - the suction works the same way as it does through gravel and just lifts any solid waste fom the tank bottom.
Other than that, more filtration is usually the answer.
Personally, I recommend external/cannister filters, but they're not the cheapest. The good thing about this type is you can position the intake tube close to the bottom of the tank, but even then it won't necessary lift all solids from the bottom but they do keep solids moving around the tank and are more likely to get sucked up back through the intake. You could also just add another internal filter to the one you aready have, but the suction is not usually as powerful.
Most people on here recommend filtration that processes the whole tank at least 10 times an hour for goldfish, as they are soo messy.
So for a 20 gallon tank, which is about 75 litres, you need a filter or filters that will turn around at least 200 gallons or 750 litres an hour. I've just checked and the Fluval 2 plus you have does about 400 litres an hour, so you need another 350 litres an hour to make roughly 10 times filtration on your tank, if you see what I mean.
At this point it would be good to double check exactly how many litres your tank is. You could also just get another Fluval 2 plus to make up your 10 times and then that would do about 800 litres per hour, over what you need. Whether you decide to go bare bottom or not, it sounds like you do need to add more filtration anyway. A single fluval 2 plus will be working very hard to biologically and mechanically filter a 20 gallon tank.
The most popular filter types in the UK and most readily available tend to be internals like you've got and external cannister filters. You may find other types like bio-wheels and hang on backs but they're not so readily available here. Personally, I would avoid any type of air powered filter system with a goldfish tank and definately not an undergravel filter (which you coudn't use on a bare bottom anyway for obvious reasons) - they tend not to be up to the job with goldies.
Fluval are a good affordable brand, as are tetratec, there are many others available in the UK.
Hope this helps a bit