But the problem is that the visa concession comes with the rather unhelpful caveat....

Please note that this concession is operated at the UK Border Agency's discretion. Our officers may therefore prevent you from using the concession. If you are unsure whether you meet the concession's criteria, you should get a visa before you travel.

Also, Jet Airways will get a hefty fine if you are denied entry into the UK, so they have to make a judgement call on whether to allow you to board.

Would be far easier to book a same day connection, or transit through somewhere that you don't have to go land side (like Amsterdam).