I started mtb racing back in the early 90s on a rigid '93 Klein Attitude, I then moved on to a 22.5lb '95 Pulse hardtail and stuck with that for ten years before moving in to CX. I want to finish my racing days back in mtb. As you can tell I am well out of date with stuff but I have been looking into what is out there and quite like a Scott Scale or Spark. I've watched the UCI WC videos to get an idea of what the pros use and there doesn't seem to be any hard rules as to whether hardtail or full suss is a better choice even with the rocky stuff they have to ride these days. I know uk race formats have changed too since I last raced mtb so I cannot decide which is better suited to uk courses. I hope to do the Southern MTB series and MSG Eastern. Help!
I don't think it matters that much. Get something you like riding. But I would recommend a dropper and 2.2 or 2.3 inch tyres. Personally I like FS and would ride it even down south, because it lets me sit down and pedal more, and I'm used to the body language required. But you might prefer hardtail if you're more used to that.
Short travel carbon 29er. Will be quite different to what you’ve had before, and different is good. It will also be fabulously capable of more than just the odd mtb race.
You’ve done rigid and cross, time for some fast, light, comfort.
Speaking as a man who's NEVER done a xc race:
Do you know any of the race venues & courses already? Get a demo or two and wallop round; I have a feeling that WC courses are a fair bit more demanding than many/most/any (I dunno) UK ones and that "any" xc bike would do. That means you can pick what you like
(I'd have a short travel FS too)