If I take a US lamp back to the UK, can I buy bayonet fittings to convert it? Or do I just garage sale it before I go.........

Sell it.

Depends on the lamp and how much you love it. You could buy a small step-down transformer and plug the lamp into that in the UK (take a good supply of bulbs home too). Rewiring may be easy but often isn't, depending on how the internal wiring in the lamp is done and whether grounding is an issue. If shipping isn't and option take it home and ask an electrician. If it's not a family heirloom or a tiffany shade it will be cheaper to trash the lamp and buy another in the UK.

http://www.thelightbulb.co.uk/faq_bases.php

US type screw in bulbs are available in the UK