Actually, VAT is included but only for UK customers, the UK being their main customer base. Clause 11 of their T & C states "All prices shown are inclusive of VAT". The reason why this applies to customers in the UK but not those in other member states of the EU is a little complicated. Generally speaking, Import VAT is only added to goods imported from outside of the EU over a certain value. You mention Eur 25 being the value for Germany. In the case of the UK it is £15. The principal difference, however, concerns the status of the Channel Islands relative to the UK. They are not part of the UK and cannot, therefore, be part of the EU. Thus, you in Germany are allowed to import goods from the Channel Islands, below the value of Eur 25, without Import VAT being imposed. However, I in the UK cannot import anything from the Channel Islands, irrespective of its value, without Import VAT being imposed! It is for this reason that Zavvi include VAT in their prices for UK customers.
This was not always the case. In fact, prior to the latter part of 2011 (I think it was November but wouldn't swear to it), I was able to import goods, below the value of £18, from the Channel Islands without the imposition of Import VAT. The status of the Channel Islands, as not being part of the UK, had created a legal loophole, whereby a number of UK companies had set up bases of operations there which enabled them to sell goods direct to UK customers, free of Import VAT, provided the value of the goods remained below the value of £18. The first was play.com (Jersey), followed by others including hmv.com (Guernsey), sendit.com (Guernsey) and dvdpopcorn.com (Jersey). Amazon.co.uk even had a Jersey based subsidiary.
However, all good things must come to an end and, due to the effects of the recession, there was intense lobbying from mainland retailers that the Channel Island based companies had an unfair trading advantage, in that they were able to undercut their prices by legal tax avoidance. As a result, the government reduced the Import VAT threshold for goods from outside of the EU to £15 and specifically excluded the Channel Islands from any such threshold. Henceforth, all imports from the Channel Islands, irrespective of their value, would be subject to the imposition of Import VAT. It was this act which ultimately led to the demise of hmv.com and the withdrawal of play.com from direct sales, which brings us back to Zavvi.
Zavvi's principal customer base is the UK so they have an arrangement with UK customs (HMRC), whereby Zavvi will collect the VAT from the customer at the point of sale and remit it to them. This is why the retail price is inclusive of VAT for UK customers. If they didn't do this, every package shipped to the UK, irrespective of its value, could be intercepted by Customs and surcharged with 20% Import VAT, not to mention a Royal Mail handling charge of £8 on each and every item! They would not be long for this world if they had to trade under those conditions.
As to why Zavvi do not remove the VAT element of the price when the item is being shipped overseas, only Zavvi can answer. I would surmise that it is for the sake of simplicity that the retail price is uniform, irrespective of where the customer resides. Amazon's sites clearly have sophisticated software which enables them to not merely remove the VAT element altogether when the item is being shipped outside of the EU but also harmonizes the amount of VAT levied when shipped to other EU countries where different rates of VAT apply. Thus, when I order from amazon.fr or amazon.de, the VAT I pay increases because their rates are lower, whereas when I order from amazon.it or amazon.es, the VAT I pay decreases because their rates are higher. This is probably beyond the capability of Zavvi's system which cannot even differentiate between an item which is genuinely 'sold out' and one which has yet to be made available for order. Also, as others have already pointed out, if ever they were to remove the VAT element on sales to overseas customers, you can rest assured that overseas shipping costs would rise to cover the loss of revenue since they currently bear no resemblance to what Zavvi actually pay.
