I don’t think they’ll open KHs any time soon in the UK because there is no way that there is enough room in the KH for people to stay 2m or even 1m apart. Even when the risk is very small I think Watchtower will be reluctant to instruct people to return to the KH in case older JWs catch the virus and die and Watchtower gets the blame. Who knows, it could be as long as a year or two, if and when a working vaccine has already been rolled out, before JWs return to KHs.
Some churches are already holding services in the UK, but mainly in large church buildings where attendance is low and spacing out is easy. Even in those cases many older people (who comprise the vast majority of church goers in the UK) are reluctant to return.
Church going has been in decline in the UK since the 1950s with the rate of decline increasing in the 1990s. On top of that established trend, when the dust settles, I imagine the 2020 pandemic will have a significant negative impact on churchgoing in the UK. It might impact the viability of some shrinking denominations such as the Methodists, Unitarians, and the United Reformed Church. It won’t extinguish the established and national churches, the Church of England and the Church of Scotland, but it may bring the day closer.