Fresh Yeast availability in the UK
There are many posts on TFL that recommend trying to source fresh yeast from your local, large supermarket bakery department - when it is a bakery not a tanning salon for partially baked loaves. Also, Richard Bertinet will supply good fresh yeast but either in 500g blocks or on a regular "plan". There are also some specialist suppliers such as NiceisLife (Italian produce http://www.nifeislife.com/  ) who also sell small packs of fresh yeast.
I have now found that the online supermarket Ocado also supply fresh yeast ( http://www.ocado.com/webshop/product/Kronjast-Farsk-Jast--Fresh-Yeast/79107011?prevPageIndex=1&from=recommended_items&tags=%7C20000&parentContainer=AL...  ) and they say it can also be frozen. I regularly freeze the yeast I get from M. Bertinet in plastic wrap and then a strong, thick freezer bag so would recommend a similar treatment for any other types of fresh yeast. Ocado recommend using double the amount of defrosted yeast compared to fresh.
I appreciate that Ocado may been seen as expensive but they do a price match on branded goods to Tesco so it isn't that bad - unless you get sucked into buying some of the more unusual items that they supply, like I do - the fresh Girolle mushrooms, sour cherry juice and Italian rocket, to name but three, are great :).