Luke, for that amount you can probably benefit the most by getting wheels from reputable Chinese brands (eg LightBicycle or Farsports), also cause you can build them as you want (and at your weight you may want to consider a few additional spokes, unless you're having no issues currently). A set of wheels could set you back some £600 to £900 depending on hub choice etc.
There's two threads in the "Wheels" section for you to learn more on these.
Other than that, in case you don't have top level tyres, please consider upgrading those. The difference between having basic stock tyres with butyl tubes and top range allround/race tyres with either latex tubes or tubeless is *huge* and you *will* notice it. Wheels and tyres, for a relatively small amount of £££, will make a difference that is far greater than what the frameset alone would do.
I haven't thoroughly read all answers, but another thing you may consider is optimising your position. Being in an aggressive position may be easier for you as you probably have plenty of core strength and flexibility, and especially on the flats it will help greatly. A set of slightly narrower aero handlebars will help greatly in both being more aero in itself, and also allowing you to get more aero, so this is also something to consider that would yield a great benefit for relatively little money.
You definitely have plenty of room for improving your FTP, and probably positioning can also yield great gains, but you can nonetheless treat yourself to some bling, especially if it helps you liking your bike more and wanting to ride it more - just leave the frameset for a while and focus on bits that are more important (IMO, tyres, handlebars, wheels, in this order)