I know it's a bit late now, but you might like to use either
Sedbuk Boiler Calculator or
EST online boiler calculator to find out what size boiler you really need.
Please note:
Dimensions are internal
Room height is floor to ceiling
A mid-terrace has two external walls
A semi has three external walls
A detached has four external walls.
You could also use
Stelrad Elite Catalogue to find out the output of your radiators.
You are quite right to think that a rad running at 75C will give of less heat than one running at 82C. The output of a radiator is determined by three temperatures: Flow (water in), Return (water out), Room. For this reason all rad outputs are quoted to a British Standard, which specifies temperatures of 75C/65C/20C.
The principal reason for running boilers at a lower temperature is so the return temperature is below 55C, if it is the boiler will condense and the efficiency will increase; this is compensated by installing oversize radiators. You also use less fuel.
At the temperatures you quote of 62C/51C, the radiator outputs will be about 65% of the manufacturers figures. This is nothing to worry about, it's due to the higher outside temperatures - heat required is directly proportional to the difference between internal and external temperature. The boiler has sensors to measure the flow and return temperatures and adjusts the boiler output accordingly.