The 20% thing was approved about a month ago, and it's been a bureaucratic mess. I applied to it on the first day of being on the law, and as of today I'm still waiting for approval. Apparently their systems are not prepared for this discount and they have to be applied manually. Of course you don't have to wait, you can pay full rate and then claim the difference, but that could take months.
If you create a SL, you must designate a legal administrator, and the cheapest way is being an "autónomo".
Edit: about creating a Ltd in the UK, if your revenues are more than 50% from Spain, you will be fined by the Spanish IRS for capital flight. It's not easy.