Nobody who accepts payment in multiple currencies is obligated to give the "official" exchange rate, which is the rate banks charge one another and not a retail figure. In fact, most do not - even your own bank. PayPal only promises "competitive" exchange rates.
One way to mitigate this is to tell PayPal to use the vendor's currency, not yours.. Yes, you can do that. It'll then be your credit card company that charges the conversion fee, not PayPal. Credit card companies typically charge a little less, but because there can be a delay of days or even weeks before your CC is actually debited, the exchange rate can significantly change between the time of purchase and the actual charge. You could even end up paying more.
If you type "Euro to Dollar" or "Pounds to Euros" in your Chrome search bar, you'll get Google's conversion utility. This shows a graph of the currency's history and you can get an idea of whether or not a delayed calculation is likely to benefit you. Those graphs currently show that the pound has been steadily rising against the Euro, while the dollar-to-euro rate has been fairly constant in recent months. So for U.S. consumers, UK-sourced products are getting more expensive, while EU-sourced products remain a good value.