E.g. In 2017 SNP won 35 seats for 3% of the vote (and way way too much press) while LD won 12 seats for 7.4% of the vote. https://www.bbc.co.uk/news/election/2017/results
And it was much starker in previous elections (e.g. when UKIP took 13% of the vote).
There is also almost no point in voting under FPTP unless you live in a marginal constituency and only 20%-ish could be called anywhere near marginal. Therefore vote swaping as suggested by @GuyD73 is a good way to attempt to place a meaningful vote.
I did this at the last election via this site: https://www.swapmyvote.uk/ There may be others. I was able to communicate with the voter in another constituency (where my chosen party had a chance) and agree that we would vote for each other, thus making both our votes more meaningful. The drawback is that you have to trust the other person! But at least there is a hope one's vote could count for a change.
![Surprised :o](./images/smilies/icon_e_surprised.gif)