Are PRSAs available where the individual decides on which investments to buy?

If yes, which provider(s) facilitate this?
and
Is there any prohibition on individual cryptos?

Independent Trustee Company (ITC) offer self-administered PRSAs. There are others but ITC are the biggest of the bespoke product providers. Standard Life have a self-invested fund as one of their fund choices, which links to a Stocktrade trading platform.

I don't think Standard Life facilitate individual cryptos. Don't know about ITC.

You can't purchase crypto through a pension. How will it be held in the pension structure if its all on blockchain?

There is a BitCoin ETF available, BTCetc - Bitcoin Exchange Traded Crypto