I've not long since started my quest to 'spot' every unit in mainland UK (
shameless plug), so hopefully I can offer some starters for ten.
As Harbornite says, all you need if you're purely after logging units/locos, is numbers. Some are easy to get, such as EWS 66s or 150s, some are an absolute devil to observe, notably 220s, 221s and 390s!
I have a slightly convoluted approach to noting down numbers, which varies depending upon where I am. If it's a quiet spot then I simply log them directly to my spreadsheet on my phone, which automatically flags duplicates (I'm not interested if I've seen it before). If it's somewhere hectic, or the phone battery needs conserving, then I use a wee notebook, and transfer numbers across to the spreadsheet when time then allows. The latter method does throw up more duds, but I can at least filter them out later on.
To track overall progress I heartily recommend
Trainlogger. I've paid for full access, but even the free version is good.