Who knows? UK is tourist-friendly country, Russian state nor can, neither wish to control every tourist there. In 2017 there were near 190 thousand of organised (by tourfirms) touristic visits directly from Russia to UK, most popular of them 8 days long.
Российский турпоток в Великобританию практически вышел на уровни докризисных лет | Туристический бизнес Санкт-Петербурга
But it is only part of touristic flow - many Russians buy tickets by themselves, or visit UK from the other countries.
Right now, most popular one day-long tour is London and Salisbury.
World. Salisbury's Popularity Has Grown Among Russian Tourists
Google say that there are four major visitor attractions, attracting more than
500,000 visitors per annum, within the Salisbury locale.
If Russians are near 5% of them, it is near 25 thousands Russian visitors every year, near 70 Russian tourist a day, much more at holidays and weekends, much less other days.
Just a statistic.