Most EU P2P platforms require you to make at least the first transfer from your UK bank account to satisfy KYC checks. Some UK banks charge quite a lot for an EU transfer, which puts you off if you just want to top up 100 euros to try it out. Mine charge £12 or so for each transfer, before any exchange rate spread.
I have been using a smartphone app called Revolut to make these transfers for free. Afterwards you can continue to use the Revolut app to exchange and transfer the money, or something like transferwise.
Or you open a transferwise borderless account where you get a EUR IBAN, transfer fee is 60 cent, receiving free. Or you open a paysera account, the EUR account with individual IBAN is free including 30 free transfers/month. Paysera's exchange fee is far worse then revolut or tw.
As Neil points out, revolut might be the cheapest way.
I've been running the numbers to see whether a Revolut premium or Transferwise borderless is going to be the cheaper option for me (c. €30k pa being invested into EUR platforms with monthly interest withdrawals to UK bank account).
For me there's only a tenner or so in it at both platform's current approach to rates. Having bought BTC through Revolut, and being none too impressed by their approach to crypto rates, I'm forming the impression of a company who prefers a certain opacity around their business model in favour of marketing headlines and convenience - they do state their business model is predicated on good rates, but it's also based on convenience and shouting "free" a lot, so we'll see. TW are certainly more upfront and transparent about their charges. However, I probably will use Revolut premium, mostly for the fringe benefits and the possibility of saving money if I choose to withdraw capital at any point over the year. Still, I'll be keeping one eye on their rates.
For now Revolut would certainly be cheaper for lower volumes, if you're not intending to convert more than £5k a month (after which a 0.5% fee kicks in) and are happy to leave conversions alone on weekends (where a 0.5% mark-up on the Friday rate is charged to "protect their position").
"[Capitalism] has drowned the most heavenly ecstasies of religious fervour, of chivalrous enthusiasm, of philistine sentimentalism, in the icy water of egotistical calculation." Communist Manifesto