and most of us in the UK nowadays think in metres
I'm not so sure about that statement. Weights maybe, but surely not distance and speed.
All signs in GB must be shown in Imperial, it is illegal to have signs showing only Km or metres. You usually find height restrictions have both Imperial and metric and maybe width and weight restrictions, but that appears more for the benefit of foreign lorry drivers.
If you are for example driving from London to Brighton, the road sign will say 52 miles not 83Km, countdown markers on motorways are in yards. Check a speed limit sign e.g. 70mph and you use your speedo to check for 70mph.
I still think in Miles, yards, feet and inches and your preference appears to be Km or metres but I'm sure that if we were to start side by side and you were asked to 'stride out' a distance of 1500 metres and I had to 'stride out' a mile, I guarantee neither of us would be right. I doubt we could even guess at 50 metres or 50 yards.
I've travelled many miles (km) in France and never had a problem with countdown markers being in metres, after all a yard is 36 inches and a metre 39.25.
I still use my speedo in mph, when in France, having done a quick mental calculation from kph. e.g. Take 90kph, I ignore the nought and multiply by 6 which gives me 54mph. It works all through the range and is only 1 or 2 mph under the actual limit.
But getting back to your question. If you select Change preferences, scroll through to Page 5, select Set units.