I truly have every sympathy for the Afghan people and we (and other nations) should support as many refugees as we can.
However, it may be that 20000 is all we can reasonably take 
Housing in this country is a massive issue. There isn't enough social housing as it is. Nor enough private rental property.
These people need more than houses too. They will need access to GPs, dentists, midwives, health visitors etc.
The children need school places and it isn't a case of just stick them in a class and off you go. Many will have limited English so need additional support with that, or SEND needs. They will likely have covered a different curriculum.
I genuinely wish we could have as many refugees here as need a place but that is most likely not feasible.