Although our opinions of 'formal' and 'informal' differ on either side of the pond, I would certainly go with the others and recommend long sleeves.

The thought of attending a wedding sans jacket is almost more than my system can take, but I know that's the sort of thing you guys do over there!!

Comfort is important, but so also is the show of respect you have for your hosts and those around you. We cannot, nor should not, put our own comfort above all else, at least not under such circumstances.

Whatever; enjoy that wedding.