In reply to Andy Hay:
If you just want the absolute longest (including stuff like the girdle of stanage and various other massive horizontal undertakings), have a google, I've seen this exact question several times before on UKC.
If you are looking for cool routes to get motivated for, I'd add a grade range, an accessibility range and maybe an idea of how much adversity you will put up with because several of the longest routes in the UK are on remote islands (Shetland, Orkney) that take 2 days to get to, are filled with detachable garbage, decorated with vomiting fulmars and in the mid E-grades or harder.
There are a fair number of mountain routes of around 300m or a bit more in Scotland, I'd suggest that Aaron, Shelter Stone, Ben Nevis North Face, Carn Mor and triple buttresses on Ben Eighe are some of the best places to go for long routes below E1.
The longest route is definitely not going to be the most worthwhile.