My Britannia tender came with a realistic drawhook - see www.britanniabuilder.co.uk/Kit4heatinghose.jpg - but the instructions said that this musn't be used for towing, which I can understand. They said that an alternative fitting for towing would be provided in a later kit, but this never materialised, so it's one of the small number of items missing as a result of the demise of Modelworks.
Are there any standard dimensions for the clevis used to attach to the driving trolley, or should I just check what people have at my local club? The scale drawhook is fitted onto an M3 thread on the end of a 5mm rod through the slot in the buffer beam - this seems rather flimsy for towing, so I guess that I should substitute a rod with an M5 thread into the clevis. The scale drawhook is spring loaded with a stiff spring behind the buffer beam - would this be of any benefit when towing?
Perhaps just for guidance - the rear coupling for Don Young's Black Five has a 1/4" (6mm) diameter round section that fits through the rear buffer beam and the clevis is drilled No 10 (4.9mm) for the pin. The slot in the clevis for the coupling bar is 3/16" (5mm) wide. Hopefully your Brit should be able to pull at least 20 people so you need a really substantial coupling.
It never ceases to amaze me when I see the flimsy couplings and pins (often just an ordinary 2BA steel bolt!) that some drivers at our club use to connect to the driving/passenger trollies. A disaster waiting to happen. Even on Helen I use a 3/16" dia hardened steel pin. I think Maidstone Engineering (?) sell the pins with a safety clip to stop them coming out.
Check that the loco to tender coupling link is strong enough as well. They have been known to snap with rather scary results!
As you suggest have a look at what other drivers at your local club use and seek their advice. Some clubs now insist on safety chains between the loco and the driving trolley as well as the coupling link.