At low speeds and short distances (eg 40 mph and less than 20 miles), the driveline will probably be OK. But there is a small bearing in the middle of the gearbox that will turn all the time (unless of course you disconnect the driveshaft). It normally gets lubed by the countershaft splashing the oil all over; but the countershaft will not be turning.
I tried flat towing my TR3A once by tying the tow bar into the bumper mounts (in place of the bumper), and didn't like the effect it had on the front apron. There was obviously a great deal of force causing it to flex sideways, and some damage to the sheet metal at the junction with the mounts. I wouldn't do it again without a bracket that tied all the way back to the frame itself, and not into the front apron or bumper mounts.
Easier to borrow or rent a trailer or dolly, IMO. I'd own a trailer if I had room to store it (and I've been thinking about designs for one that would fold up and roll into the back yard).