The simple answer is yes they can be, so long as the combined power they are drawing does not exceed 13 Amps. 13 Amps is the maximum power that can be put through a conventional UK fused plug socket.
13 Amps equates to 3 kilowatts at 240v, so for example, you could have two 1.5kw heaters, put them into one junction box with a single plug attached.
PLEASE BE AWARE:
We recommend for safety purposes that any kind of wiring be done by a qualified electrician.