Several people have commented that a 60 watt heat tube would not be sufficient to keep your trailer warm, and I agree.
I have heard a couple campers here in the American northwest, which has a similar climate, mention they've had good luck with 500-700w oil-filled radiators like this one.
Lynne and I have used four types of heaters and an electric blanket at different times. One is a built-in propane furnace
for times when we don't have electrical
hookups, another a "cube heater
" for times that we do. We also have a Wave 3 catalytic heater that uses propane
and no electricity, but which requires a certain amount of care when in use.
We've also had good luck with "Buddy
" heaters from Mr Heater. They don't work well on mountains over 5000'/1800m high, but since mountains that high are rare in the UK, that shouldn't be a problem for you. The nice thing about the Buddy heaters is, unlike the Wave 3 heater, they have oxygen
depletion sensors that prevent them from sucking all the oxygen
out of the air in your trailer, so they are safer to use.
Best of luck to you.