According to my records no B788's were used for this route during 2016, only B738's. Although HER can handle B788's the new long range flight destinations and the demise of the B763's would preclude there use on short range flights.
Thomson have two 788s on short haul during the summer, but it's not unheard of for a longhaul configured one to be used on short haul as well. LGW-HER was operated by a 788 on some dates this summer.