The short answer is that knowing that the euro is weak against the pound doesn't tell you the answer to this question, as you don't know when a shop priced its item. However, in general when one currency gets stronger against the other it is better to buy in the weaker currency.
Basically imagine the item was priced one month ago at £10 and (e)15 at a time when these values were equal, i.e. so £10 costs (e)15. Clearly in this scenario it doesn't matter which currency you buy in - the value is the same in both.
Imagine that over the last month the euro has got stronger against the pound, Now, (e)15 buys £11. However, the shop has not updated their prices to reflect this change. So the item is still £10 or (e)15.
Disregarding the cost of exchanging money, it is now better for you to use your euros to buy the item. If you bought it in euros you would still be spending (e)15. But if you bought it in pounds you would be spending about (e)13.64.
However, in the case of any given website, you don't know when they priced the item (or what other factors went into pricing). So you can't tell just from knowing that the euro is currently strong against the pound that it is better to buy in euros. You have to convert the price in pounds into euros.
You also have to work out who is converting the money for you (your bank or the website) and whether they will charge a commission/fee. For low value items, a fee to pay in pounds probably means its best to buy in euros.