Many drivers new to EVs get worried by these mileage estimates but the figures are based on a few criteria, eg. your driving style based on your last journey and as you have discovered, the temperature. The figures you quote are well within the range expected at this time of year in the UK and I don’t believe you have any cause for concern. My own range has dropped from an overall average since new of 3.2 miles per KWh to around 2.7 just now, so much the same as you are getting.
Setting the range manually would not serve any purpose even if it was possible, as it would not mean you would achieve that range. It would be like having a range estimator in an ICE car and setting it to 10,000 miles then saying that is how far you can get on one tank of petrol.
You can improve your range particularly in winter by preconditioning your car while it’s connected to the charge point, which saves using the drive battery to do the heating and charging just before you leave also warms up the drive battery.