I don't believe there's any stipulation regarding trailers having a fixed drawbar. I had a quick scan of the trailer braking directive (71/320/EEC) and couldn't find anything in that.
Additionally, I used to sell plenty of Brian James Micro T, mini car transporters, which had folding / lifting drawbars to save on space.
I think if you have a type approved trailer (unlikely in the UK, as this stipulation is still being phased in - any saddos out there, like me, have a shoofty here for more details: Trailer Type Approval
) then you probably shouldn't muck around with the design of the trailer.
However, if you have a trailer manufactured by a company in the UK (that has now fallen outside of it's warranty period) I don't think it's unreasonable to make modifications as you see fit. Where do you draw the line? You have to go back to the manufacturer to have a spare wheel fitted? Or that really nice ally box you had fitted retrospectively has now made your trailer illegal because you drilled some holes through the chassis?
I appreciate it's not quite the same as fundamentally changing the design of the trailer and adding a folding drawbar, but you appreciate my premise, I'm sure.
So legally? I can't see anything that says you can't (perhaps Mike at SW Rib, or the other trailer chap on here knows differently? I'd appreciate any input you guys may have).
Is it safe? Absolutely, if you know what you're doing or have someone do it that knows what they're doing.
All the best
PS. Andy, if you want a chat, feel free to drop me a line - 07921 853190