If I buy an iphone 5 (GSM version) from Apple US would it work in the UK?

I want to use it with Vodafone in the UK.

anyone else done this, any issues with this?

IF…
You can get an unlocked / SIM free one - yes.
They cost some 800 USD.

NB most of the prices you see are network locked models and on 24 month contract (see the notes on the apple site - they say 'needs a contract arranged separately'). The apple shops sign you up in store.

Essentially, there's no price advantage to doing this.
and if you ship it in, there will be duty to pay (5-7%) and VAT (20%) on all of that to pay.

Any reason that you want a US one.
(there are 2 US models, one GSM/3G only, and one 3G/US-CDMA eg Verizon)

No. It would be locked to a US carrier. You'd have to get it unlocked for it to work in the UK with any other carrier.
Apple UK offers the phone, so why not get it from them instead?

I'm assuming you saw the prices Apple US has & thought it was a much better deal? Long story short don't bother trying to get it from there.
http://www.vodafone.co.uk/brands/iphone/pay-monthly-iphone/index.htm?icmp=int-vod-hpg-hb-bt2