I think you're going to be disappointed with that high limit stat clipped to the boiler flow. Depends how quick it is to react. May be okay if if has a mercury phial.
I once tried to govern a boiler, that had a very high hysterisis internal thermostat, using the Honeywell pipe stat (which was a basic bimetallic cylinder stat which clips to a pipe). The problem was that the boiler raised the water temperature faster than the Honeywell stat could react. So the boiler would overshoot before the stat became aware of it. If it set the Honeywell stat lower, it would prevent overheating, but then the boiler had to run almost stone cold before the stat would allow it to refire. Eventually it turned out that the boiler internal thermostat, thought to be obsolete, was available, and replacement solved the problem.
I would agree with others that the solution would be to fit a thermostat to your water cylinder and convert to fully-pumped. It'll be of the cheap and nasty bimetallic strip design, no doubt, but the quantity of water involved means that the stat will have time to gauge the temperature. You'll find a cylinder stat may not be especially accurate to the marked settings, but once you have got the setting where you want it to be, it should be surprisingly consistent.