Good that you've got some prawns going, should keep the bacteria alive until you can find a kit and ammonia.
The kit that I and many others use on TFF is the API Freshwater Master Test Kit. The Nutrafin Mini-Master is also a liquid based kit that gets good comments from our users. UK seems to have plenty of good online sources, you can search with both google of course and via searches on the topic within the forum. (As Corleone says, nitrate tests are difficult for any kit, liquid type included and all shaking instructions must be followed to the letter.)
Yes, even for a small 10g you will want a small gravel vac, ie. just a small clear tube on the end of a siphon tube, allowing you to let the gravel churn up and the siphon pull water out of the tank. You can do a third or a half of the bottom surface probably before the amount you want is out of the tank (this will be later of course, after you have fish and are settled. -- although for you, if the filter turns out to be already cycled, you will be able to begin this weekly habit very soon.)
Ammonia can be difficult to find but is important for a controlled fishless cycle. Hopefully the UKers will tip you off as they have a couple good ones nailed down. Usually its a big box or hardware strore type place. Its sold for cleaning purposes but you must find pure clear ammonia. Ideally you need to shake it to see that it only forms bubbles like water and does not foam as it must not have soaps in it. It needs to not have dyes or fragrances in it and many do. Do searches on "ammonia" here of TFF as in the past some in the UK have even photographed the bottles of ones that worked ok for them, but even with this you must be wary of the manufacturer making changes to what goes in the bottle. The right thing is almost always an aqueous solution, about 9.5% ammonia in water, but sometimes the actual contents are not marked and it will only say "ammonia" or "clear ammonia."
Good luck and its important for you to be reading the pinned articles...
~~waterdrop~~