Some of the diverts may be down to the wind direction and runway alignment in different airports. Heathrow's runways run west - east (270 degrees/90 degrees); earlier the wind direction there was 232 degrees so there were 38 degrees of crosswind to take account of. Manchester's runways are 230/50 degrees, so if the same wind conditions applied as in the Heathrow case, the wind would be pretty much straight down the runway, so a lot easier to handle.
You also need to factor runway lengths into the decision. A longer runway gives more options than a shorter one. You also need to consider the geographical features around the runways; 27R - the northern runway has buildings on approach which cause turbulence. In Storm Eunice, they used 27L for landings to mitigate this.
It seems perverse that aircraft which can get off the ground full of passengers and fuel cannot land with the same conditions. Fuel dumping happens more with long-haul flights as they obviously have more fuel onboard, so if they need to return or divert they need to get the weight down to the maximum safe landing weight. Knowing the conditions today, pilots will almost certainly have carried extra fuel in case they are forced to divert.