SporeWorks offers liquid cultures. They have very reliable and excellent service.
Once you have familiarized yourself with the cultivation of mycelium and aquired what supplies you did not already have, then make your purchase... Any pleurotus species would be a great starting point. Whichever species of fungi perfecti that you chose to work with, I highly suggest starting with ones that can be cultivated outdoors during two or more seasons out of the year.
I agree with Rick, but you might want to read one of the excellent books (Mycelium Running, Growing Gourmet and Medicinal Mushrooms) by Paul Stamets about cultivation first before ordering spawn. The garden giant (king stropharia) is a real good one for beginners and can be easily grown on wood chips in a garden. Maybe you could find a local spawn producer in the UK. Winter is an excellent time to read up on cultivation, and spring is a great time to start mushroom projects in your climate. Good luck!
Stropharia rugoso-annulata (wine cap, garden giant) can be put out after danger of all frost, if placed under 3~6 inches (8~15mm) of good mulch... It can be started and expanded "indoors" ahead of time, as to have a large amount of spawn to put down as soon as possible
It can be expanded in bins to great volumes
i don't think fungi perfecti or sporeworks ships to the uk. I've been watching a lot of videos about how people grow mushrroms commercially and domestically. I bought some spores for white button mushrooms i tried to start them in compost and newspaper about 6 weeks ago it just seems like nothing is happening. so i'm trying again this time in a bucket in my basement using compost and wood chips lets see how that works out...
Liquid cultures are not very beginner friendly. I think the bag is easiest. You could add boiled sawdust or chips to your bag to extend it. Then the wedge or totem method with logs works from that sawdust, or you could go to plug dowel spawn, with a drill and wax. Agar and liquid culture is way after all that. I agree that the Stamets books are the absolute best for learning about cultivation. Mushroom people and Field and Forest are other good companies and Field and FOrest also has an informative cataolog for free.
Once you have familiarized yourself with the cultivation of mycelium and aquired what supplies you did not already have, then make your purchase... Any pleurotus species would be a great starting point.