Sylhetis arrived in Britain in two en masse migrations- one in the 60s and the other one in the 70s. Those who came in the 60s hold quite strong pro-Pakistan sentiments and were quite vocal in their support of Pakistan in the '71 civil war, since secession of East Pakistan meant automatic exclusion of Sylhet as well (does anyone see parallels with the Scottish dilemma on Brexit or is it just me?) The third and fourth generations of these families still love Pakistan more than anything and I am quite happy to be part of this vibrant group
The '70s immigrants are a different case altogether. Those of whom still have relatives in Bangladesh are pro-Bangladesh and have readily accepted the Awami version of '71 for obvious reasons. But the third and fourth generation immigrants support Bangladesh as well as Pakistan. Sylhetis on the whole are more religious and conservative than Bengalis, so religion is a binding factor here. To summarize, Sylhetis are more pro-Pakistan than pro-Bangladesh. You can see our leanings on pdf for yourself
As for the joining India part, absolutely no Sylheti wants to join India. They would much rather be in a Muslim-majority country. Its part of the reason why we voted for Pakistan in the '47 referendum. Religion is very important for us