If you are sharing a download for OMSI 2, the community appreciates transparency. Always mention:
If you want to drive a right-hand drive (RHD) bus in a British environment without paying, the OMSI 2 community has developed excellent freeware projects. While there are fewer pure "London" free maps due to the massive scale required to build the city, several projects exist: OMSI 2 Add-On London - Steam Omsi 2 London Map Free Download
To complement free maps, you can download specific London-themed vehicles and repaints for no cost: If you are sharing a download for OMSI
Assuming you have found a legitimate freeware map (like Bowdenham or a small London segment), here is how to install it safely. Omsi 2 London Map Free Download