Rob the gran canaria flight will have been orignally operated a jet2 757 not a 737. The 737 doesn't have a second door half way down. Titan airways has Jet2 seats and full cabin crew from jet2 with titan flight deck. The only difference with this aircraft is it doesn't have a doors 3 and the livery is titan airways. Apart from that you wont realise youre flying on a non jet2 aircraft. The ground staff should give you the same seats that you booked (could possibly be a different row) but still in the same area of the cabin.