More than 300000 people a day get tested for coronavirus in the UK according to [1], and the testing capacity is even greater than that. This is enough to cover study participants.

I would imagine that volunteers would also be willing to get tested, though perhaps the worry is that such a requirement might cause more of them to drop out of the study.

[1]: https://interactive.news.sky.com/2020/covid-19-coronavirus/t...