There are a lot of attic conversion companies who are unsure about radiators in attics and can not always guarantee that they will work.
We can tell you when quoting and estimating whether or not we can fit the rad on to the existing system. Usually the problem is a small expansion tank that supplies water to the heating system. The way around this is to remove the expansion tank and pressurise the heating allowing water to flow to the attic rad. Rarely this can't be done such as a back boiler system but on this occasion we can get the header tank as high as possible and install a low level radiator.
If and rarely none of this works you can always install a timed electric heater with a thermostat.