You can't use your SMARTY plan's allowances in Guernsey @stephen_1731632, so you'd need to buy an out-of-plan add-on to PAYG while roaming in Guernsey.
Currently the Channel Islands, and the Isle of Man, are in SMARTY roaming band 1A. The costs for band 1A are shown on page 19 in the current price guide (online at https://smarty.co.uk/priceguide).
