As noted previously, the Society of Motor Manufacturers and Traders (SMMT) doesn't have Tesla as a member, so its general statistics don't identify specifics for the brand. However, they do list Tesla Model 3 as the best selling car in the UK (electric or otherwise) in both June and September this year, giving the numbers sold. Quarter end months, obviously.
The sales numbers for Tesla in September are about 98% of the figures that SMMT gives for “Other Imports”, so we can reasonably assume that currently Tesla makes up most of this category.
If so, I would estimate that Tesla Model 3 sales for the year to end September (UK) are very close to 23,000. Almost all cars with higher sales figures are smaller, such as the Corsa, except for the BMW 3 series (all variations) which sold 23,253. The Model 3 is not in the SMMT top 10 for the year, although the BMW 3 series is, so Tesla can't be far behind.
Even in another very odd year for car sales, this is an impressive achievement by Tesla, I think.