Yer allowed to make a profit as long as you're not buying with the intention of selling for a profit.
If you buy something with the intention of selling for a profit then you have to register with hmrc as a trader.
I've never heard of the '5 car limit' but I suppose it could be a number where hmrc could argue that you're trading. I disagree. If I had a collection of 20 cars then it's likely that I'd one day sell some and that's not a business.
If you honestly buy something without intending to sell for a profit then you don't need to register as a trader. But if you ever do sell something for more than you paid then there is a potential capital gains tax.
Except.
There is no capital gains tax to pay on machines that wear. So watches, clocks, cars, bikes 'can' wear out (even if they don't) so they are exempt from CGT.
So. Traders pay income or corporation tax if they sell machines. Colldctors do not.
The moral is if you are going to collect anything then make sure it's mechanical.
Or coins. There's no cgt on coins. So if you want to collect gold then collect gold coins. Do not collect gold bars. You will be liable for cgt on gold bars.
Etc etc.