The golden time I think was 10+ years ago which is when I started investing (maybe 20 years ago was even better) - most of mine have doubled and some tripled in value, and I always have 3 bikes on the road so the not enjoying them bit doesn't come into it. I recently sold one of my Bayliss 1098R's but wasn't really wanting to sell it as they have room for further growth and are the newest of my bikes - someone wanted it to use so I let it go. I still have 3 of them with one on the road so I will survive

There is no capital gains tax on vehicles so there is still room for good returns, you just need to pick the right bikes, preferably when they are on the bottom of the value curve. There is also the enjoyment of ownership too, better than having money in an ISA when you can see them every day.
Most of the really old, rare and exotic have already seen significant rises so with those you have kind of missed the boat although they still could rise further (above inflation) - you need ones prior to them rising at all.