I had a really hard time finding a PEI sheet as well. I ended up ordering one from the USA.
What size are you looking for? In my experience, there a quite a few sellers on alibaba that can offer you any size you wish but the shipping costs are pretty much a deal breaker. I know, none of this is in the UK but still...
Self-sourced Mendelmax 2.0-based Reprap Machine -- Ramps 1.4 & Mega 2560 -- DRV8825 (Z@1A,
[email protected],
[email protected], E@1A) -- genuine E3D v6 direct setup -- 350W custom silicone heated bed -- ABS 1,75mm -- Marlin 1.1.0-RC7 -- Cura 15.04.6