Land value isn't really a concept most people think about in the UK, so it will be difficult to get an accurate value.
Your proposal of
(property value - rebuild cost) seems like the best option, and will probably be good enough for US tax purposes. Getting an accurate rebuild value may be hard as ideally you would ask a surveyor for a precise estimate, which itself would cost money. For a rough value you can use this online calculator.
You can also get a very rough idea of land values by searching for land for sale, but there isn't all that much of it that would help with the value of a single plot: most property development happens on larger scale sites.
This site does have a few listings for Merseyside which might help, but obviously land values could vary a great deal by locality within a single area. You should be looking at plots with "Detailed planning permission" for "Residential development", as your own land would fall under the category. However there are only a very few plots listed, and there might well be circumstances specific to those plots that make them atypical.