How difficult is it to swap left to right and vice versa on xtr brakes without needing to rebleed them?

I can get them euro style for 10 less than UK spec, but it is not worth the 10 saving, if I end having to bleed them.

On the other hand, how well bled do shimano brakes come from factory anyway?

It’s a dawdle. I do it several times a day. Mount them, keep the bike upright, unscrew at the lever end (don’t squeeze the levers) and re-fit.

Shimano come properly sorted. On older models, I like to overfill mine to move the bite point out though.

Isn’t it fairly likely you’ll have to shorten hoses anyway so you’re going to be messing about either way?

As said above, you’re very likely to need to shorten them,do it at the lever end, and purge the master cylinder of bubbles with the bleed bucket (£3.99) and some lever flicking. Worth doing even if you’ve not touched them, it’s surprising how much air comes out, and how much firmer the lever can become once you’ve done it.