Originally Posted by pauldb

You'll need a visa of some sort if you want to leave BOM airport. India does issue transit visas but these are valid 15 days from issue, so I guess you won't be able to do this in the UK, and will therefore have to go to the SL/Nepal offices. I'd still be wary as to whether they'd issue a transit visit when you have an active tourist visa, effectively as a work around to the 2 month rule.

You'll probably be ok but I'd still think the best option is to take all your paperwork to VFS in the UK and try and get your tourist visa specifically annotated.

(The are now suggestions that immigration officials are a bit more lenient on this for neighbouring countries, but you also have to convince check-in agents who may be even more unpredictable.)