John, the kind of pump system you are talking about is used quite often in drag racing. Indeed, the Stewart pump was designed for exactly that. I am not sure what is available in England, but in the US, speed shops like Jegs and Summit have parts for replacing the stock water pump. Keep in mind that drag racing has much different cooling demands, so, while some components may be useful, be careful of just bolting on a drag solution.
It would appear that your cooling system was adequate when there was not much of a load on the engine. (Presumeably, in the traffic you were probably in lower gears at lower revs.) But, when you got on an open motorway and got the revs up, the system could not keep up. This sounds like inadequate water flow. This could be caused by a number of things. To much air/steam in the system, restriction in the pump relocation plumbing, inadequate drive to the pump or overdriving the pump and causing cavitation (were the pulley sizes changed?), to small or plugged radiator, inadequate plumbing to the radiator.
There definitely needs to be a bleeder line from the top of the radiator back to a header tank. I don't think an isolated header tank in the front would help much. You also need a way to relieve air pressure from the high points in the system when filling and in operation. This is generally somewhere around the thermostat and top of the radiator. It is not always enough to have the header tank (usually connected into the water pump return and the bypass on the thermostat housing) higher than the engine. I cant stress enough that you have to make sure you can fill the entire system with coolant and you really ought to have a way that any steam created can be bled off to the header tank. There are stories of shops jacking GT40s up head high in the rear to burp the system.
If you have not read the information at the Stewart Technical Support page (link in my previous post) I would highly recommend reading everything that is there. It is VERY good information on cooling systems in general.
There are control systems that vary fan speed based on temperature and I am sure these could be used (possibly requiring modification) to control a pump as well. I think you are on the right track in looking for another means of relocating the pump. To often "home made" solutions aren't engineered properly. I am not sure that I wouldn't put a pump like the Stewart pump inline somewhere and let the thermostat control the flow. If you do electronically control the pump, be sure to provide yourself with an override to allow full power to the pump manually. Bye the way, with number of options available these days in Ford water pumps, I have to believe you could probably find one that would fit. It may require a diffent timing chain cover as well, but it would probably be worth it.
There are far more experienced Ford engine builders out there than I am. Come on guys, lets give John some ideas to solve this problem!