It's actually quite a tricky question because right now your system is well balanced. The easy answer is to upgrade the speakers. That's going to make the biggest change, but whether it's the right change is a more difficult question to answer.
The problem is throwing £250 at another set of desktop active speakers could well highlight the limitations of the turntable. However, if you throw the cash at a turntable then with your current speakers it might not seem like you've made much of a change even though you'll have a platform for a better cartridge and superior phono pre-amp.
Coming back to the speakers, active desktop speakers offer decent value, but you're always going to be limited by their size and maybe even where they have to live.
One of the most noticeable benefits of going for better speakers is where the cabinets allow for a bigger bass driver and more bass both in depth and in quantity. Changing from the 1280s to say a pair of larger standmount speakers the size of
Monitor Audio Bronze 2 would show you the bass and also the quality across the rest of the frequency range that smaller speakers can only hint at. However, trying to put those bigger speakers on a desktop or stuffed in to the corners of a room just won't work. As well as thinking about an upgrade of the hardware, you also need to consider what's a good match for the space you have available. There's no information in your OP about where your gear lives and what restrictions you have about placement.
If all you want to hear though is which speakers to buy next because the money is burning a hole in your pocket then Edifier Studio R2000DB. Overclockers has a good price, they're just waiting for stock
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