that is an issue you need to take up with Steam. PWE simply advise them the price i believe. Your transaction is with Steam.
If you purchase directly from PWE using paypal for example as i said above you pay somewhere just over 30 GBP which at current exchange rates is about right.
for the purposes of purchasing Zen PWE treat the EEC as one zone and so all purchases are in Euro. But i believe Steam treats UK(GBP) as seperate from EMU (EURO) countries, which depending on your point of view both at technically correct. UK is part of the EEC (European Economic Community) but not part of the EMU (European Monetary Union).
What seems to be happening here is that possible PWE has advised steam that zen from europe costs 45 EUR per 5000 and because steam treat uk and other european countries differently they have inadvertently applied it as 45 GBP on UK purchases as opposed to the GBP equivalent of 45 EUR.