Botanical Interests sells seeds of that strain on line, but I am uncertain if they can ship to the UK. They may be able to suggest a source over there, though.

pop 'Heliotrope marine for sale' in your browser. Google list several companies selling it .

welcome to GoY too Aitch :o)

I've grown Heliotrope dwarf marine this year from seed. doing very nicely too. Soon need to go into 3in pots.
Got the seeds from Wilkos.