as a newbie ebay seller, what happens when you have only specced UK shipping and overseas bidders (Poland in this case) bid on an item? Do I cancel their bid or give them an opportunity to sort their own shipping? We are talking about a bike frame plus bits so the shipping cost is not going to be just a few pennies more.

I would avoid 100% – just cancel their bid and point out it was a UK only sale.

Do I cancel their bid or give them an opportunity to sort their own shipping?

That's what I'd do.

Check your listing settings…you can set it to UK bidders only

I am sure you can set it from the advert, I would avoid Poland like the plague just stick with Uk.

Cash-on-collection for Poland. And especially Nigeria. 🙂

Funny, but before he realised, my Brother-in-law sold his Freelander on Ebay to someone from Poland. Chap paid a deposit via PayPal & said he'd collect.
And collect he did, he flew over, visited some relatives & drove the thing back to Poland.

advice taken, thanks for the help.