The UK based "Law on the Web" wesbite comes right to the point.
"Older cars that do not have seatbelts:
- Do not need to have them fitted and worn, but cannot be used for carrying
However if seatbelts are fitted in older cars they must be worn."
The above is supported by this UK Department for Transport web page.
"THINK! advice - seat belts & child restraints"
You must wear a seat belt if one is fitted.
Which vehicles does the law affect?
Front seats of all vehicles, including vans, coaches, buses and goods vehicles
where seat belts are fitted."
If you need any clarification, please feel free to ask.
Google search on: UK "seat belt" law
Google search on: "seat belt" law site:.gov.uk
Looking Forward, denco-ga - Google Answers Researcher