There are pro's and con's to both - obviously with a UK car you can go to look at what you are buying/test drive etc, most car's are known within the club so someone would be able to give you an opinion on it. However there aren't always too many for sale within the UK, there is only 1 car currently advertised for sale at £19.500
http://www.carandclassic.co.uk/car/C311389# (but isn't known within the club) - there is also 1 other that hasn't been formally advertised. Both grey interior, manual transmission cars from late '81.
It depends on what sort of car you are after, manual/auto, grey/black interior/ gas flap hood, pleated hood, flat hood etc - since they don't come up for sale too often in the UK when they do your choice is limited.
Importing on the other hand does open up the above options as there is nearly always half a dozen or more cars for sale within the US - drawbacks are you would be relying on pictures the seller provides to you (unless you fly out to visit in person) and/or someone from a DMC club within the US is willing to take a look at the car for you.
Import costs will vary depending on the location you are shipping from, east is cheaper than west. My car came from the west and shipping came in at around the £1200 mark, you would also need to factor in haulage from the current owner to port of departure.
Import charges when it hits the UK, import duty is normally waived provided you have a letter from the DOC stating the car was built in the UK, and as the cars are now over 30 years old they are classified as historic, as such VAT is reduced from 20% down to 5%.
You would also need to factor in if you would be willing to do the import paperwork yourself, some of us elected to use an agent to take care of this. Other costs, haulage from the UK port, essential repairs/MOT prep (uk spec headlights and rear fog light) - registering the car with the DVLA £60, first years road tax £215.
Edit - Gez bet me to it