Not exactly sure what benefit it would be to them, the 757s only have 177 seats in comparison to the A320's 174, and they'd burn a considerable bit more fuel. Perhaps if an A320 went tech they could slide in and do a rotation if there was the time remaining, but with such a low density layout, I can't see them using them in inter-European routes when they have the A320 and A321.
The benefit might be that despite the extra fuel burn they will be able to add frequency where airbus aircraft are not available or deployed on another route.
"Can BHD take a 757"
There is a really interesting idea, BHD management would give their eye teeth to get a transatlantic flight in. This would need to be like LCY SNN JFK LCY operation, doing pre clearance at SNN. Given low punt of fuel on arrival and departure I think it might work, but I'm nog sure the economics would stack up .