Generations version 8.5a is the latest version of Generations, released by the new owners Genealogy.com on the Broderbund label, and is Windows XP compatible.
The v8.5.1 patch was released by the old owners Sierra and was intended to only be applied to the USA v8, not to the UK v8.
No, the old v8.5.1 is not the same as the latest v8.5a.
If you purchase the v8.5a upgrade CD it will install itself completely separately from your old version for two reasons. (1) New versions of Generations have always installed separately from older versions. (2) Genealogy.com choose to install in a new Program Files/Generations folder rather than in the old Sierra folder. After installing v8.5a and copying your .uds data files across, you may either keep the UK Grande Suite v8 or uninstall it -- your choice.