In competing with other European destinations for tourists fro Mainland China, the UK is already at a disadvantage by virtue of being outside the Schengen Area, and thus requiring visitors to have a separate visa.

Most Chinese wanting to visit the UK would be wanting to do so as part of a wider trip through western Europe, and having to get an addtional visa for the UK is an unnecessary complication.

After the scheduled finalization of Brexit in 2021 - which will put the UK totally outside the EU as well as the Schengen area - the UK's position re visitation from China will be worse if anything.

And post-Brexit, Brits will have to file trip plans with the EU in advance and pay a 7 EUR travel authorization fee in order to visit any EU country.

So far, the only "positive" aspect for tourism of Brexit we've seen is the UK being on sale for people in the Eurozone on account of the weakened GBP.