1. Insulate with 50mm Celotex Seconds. Insulate floor, walls &
ceiling. Draught exclusion is absolutely critical. Floor is very
important re "cold calves" which are very uncomfortable.

2. If you have economy-7 metering.
Look on Freecycle or similar for a 1.8-2.5-3.3kW storage heater
(9-12-18-24kWhr). Connect to timer matched to your E7 times, ensure
battery/capacitor backup of timer and decent contacts (Sangamo Digital
Powersaver is £10-25).

3. If you do not have economy-7 metering.
Background heating by 500W frost-heater or dessicant dehumidifier
(some heating, ideal for tools & paper filing cabinets).
On-demand heating by Radiant Heater at the correct height and
clearances (this can catch you out), or 2kW industrial fan heater off
Ebay for about £30.

Insulation is everything unless you are going to be in there only a
few minutes each day.

Running a 2kW fan heater flat-out for 120 days (winter), at 2hrs a
day, at 10p/kWhr = £50 for 480kWhrs.
Running a 2kW fan heater flat-out for 120 days (winter), at 8hrs a
day, at 10p/kWhr = £200 for 2000kWhrs.
Running a 18kWhr storage heater (2.55kW x7) for 120 days (winter) at
5p/kWhr = £108 for 2160hWhrs.

You can see how Economy-7 is useful - if only for background heating.
An Air Source Heat Pump (ASHP) basically goes better than Economy-7
costs per kW/hr but on-demand 24hrs a day, but the downside is a DIY
system starts at about £299. You only need one quite probably.

You will have to set up a spreadsheet with your intended use - as well
as start planning insulation :-)

Once insulated if you are working with legs under a desk for a lot of
the time, you may find closing off the desk a bit and using a 400W oil
filled radiator makes things quite comfortable. A storage heater gives
thermal mass (65-145kg of it), so can be useful although I would not
want to mix fine wood dust and a storage heater... the core
temperature is above the ignition temperature of very fine dust. The
industrial fan heaters use a MICS (copper sheathed) element rather
than bare wire, along with a "fan oven" style metal impellor and
motor, so more useful in a workshop environment.