Welcome to the Wefugees Community and thank you for reaching out to us.
Our platform and experts are based in Germany. I am afraid that we can not share much of experience and information regarding the asylum/immigration system in the UK. All I know is that the UK left the so-called Dublin system (which regulated i.e. that ususally the country which issued the visa will be responsible for the asylum case and any other EU country would reject the application) after they left the EU (you may see this website by the European Council on Refugees and Exiles for further details i.e. on the border/airport procedure which we discussed here, for instance, as well). However, I have no idea how they deal with asylum applicants holding a Schengen visa since they left the Dublin system.
Maybe other community members will be able to add some information.
All the best,