"via a large heat store (which would also be a slot in replacement for your existing boiler)."
Such a heat store would be inadequate. You may have a better chance if the heat store was of a volume such that it replaced your existing oil tank.
If I were stuck with so few options that I had to consider electric heating, I think I'd be looking at ground source heat pumps. Many heat pumps/refrigeration plants have a coefficient of performance of around
3, so you'd get 3kW of usable heat for every 1 kW of electricity consumed by the compressor motor.
However, also consider that most electricity is produced by burning oil, gas and coal. If your oil price has increased by 50% in 2 years, where do you think the electricity price might be heading? You may be better off sticking with oil, or hedging your bets by keeping the oil boiler as a stand-by to the electric.