The closest thing you're going to get to a UK tutorial on building a bus in Blender is Rhys' C200 build-along, which can be viewed in the archived videos on his Twitch channel:
I normally recommend these Blender tutorials:
44090 Digital Models | Tutorials
The first and second are for an older version but if you can get around the different button placement and aren't afraid to google as you go, they're still really good. The third one isn't relevant to OMSI so just ignore it. The final one helps to apply the first and second to the current version of Blender, as well as introducing a few more tricks.
Regarding software, Blender is by far your best bet, but other free alternatives are available in the form of ZModeller and GMax, though the latter is severely outdated, and there are other pieces of software such as 3DSMax which can be got with a student license. At the end of the day, the software you use to build the model in isn't what's important - the important thing is what the mesh you make looks like, and any of these tools will do the job.
If you would rather go down the GMax route, I've just dug up and re-posted a tutorial I made on converting from GMax to .o3d:
Tutorial - Exporting from GMax to OMSI 2
Sketchup is another option but it's not a piece of video game modelling software, it's primarily for making architectural models and the like, and is very difficult to get good results from for things like buses. It can be used for buildings, but as you have to use Blender as part of the conversion process you'd be better off just starting in Blender or using GMax or ZModeller.
Making a bus is hard work and not many people manage to get all the way through it, but if you keep at it you might pull through.
Good luck