the other alternatives are an electric heater or electric underfloor
heating, but I'm worried about the cost.

Use your central heating.

Ensure there is good insulation throughout the house and especially in the
cellar, which has massive cold bridging to surrounding land. Use at least
50mm of celotex attached to the walls, preferably more.

As a minimum, ensure rooms (except those with room thermostats) have TRVs.

If the pipework layout allows it, subzone the heating and use programmable
thermostats. This way, each zone gets its own time and temperature control.

I know it will be considerably more expensive to install underfloor
heating but could anyone give me a rough idea of the running costs ?

If you're willing to pay those costs, you may be willing to pay the costs of
converting your heating system to S-Plan-Plus with subzoned control, which
would be a grossly superior solution, will probably end up with a system
cheaper to run than before and may even have a lower capital cost.

Christian.