FFG are always reprinting it. I'm no expert, but i think TI3 generally sells well.
Their website says the expansions are currently awaiting a reprint (and they're still available in some stores anyway) so it's definitely not going away.
Have u got names of these stores in UK?
My local FLGS's in cardiff + all online retailers out of stock of both expansions (and im not willing to pay ridiculous prices).