Hey Shane, where in the UK are you based? I will have to miss the Hudson River Derby, but will be in London (May 21st) and hope that I can catch it somewhere/anywhere. Do you know if it is likely that any pubs will show the match?
I know I'm a bit late to the party (well...a whole week - I don't check this area of the forum as much as I should) but it looks like matchpint.co.uk will tell you a selection of pubs across the UK showing each MLS game. They've only got the pubs they know about, which hardly constitute a majority, and MLS doesn't show in many places, even at a time when there's not much competition from other leagues, but you should still get a few options.
I would invite you up to my home town to watch the game, which is just up the east coast rail line from London, but it's pretty small here and I don't think anywhere covers MLS hereabouts (and also I'm pretty dull company on my own).
Try checking this out closer to the time. It only seems to give you information about games a week in advance, but it's better than nothing.
http://www.matchpint.co.uk/watch-live-MLS