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most countries have tax relief when you're moving there to live - though might have needed to sort out the paperwork in advance (ToR1 form in the UK, if it's of interest).
RE: taking parts back with you - 99.9% will be fine so long as it's not a suitcase full of iphones of something. I just flew back from my in-laws in Poland with a couple of bikes, albeit my own that i'd left there a couple of years back, and had no issues with customs/charges. Also managed to fit both in one bike bag, too!
I've been thinking about this a lot lately, the 2nd hand market for bikes and parts seems much better in UK compared to Spain, so how easily could I pick up bits when I'm in the UK and throw them in my general luggage? This might make me think twice....
I know a guy from NZ who sent a suitcase of all his clothes to BCN when he moved here, but then got a message from customs saying he needed to pay a hefty import duty on his old clothes.