They based the price on what the exchange rate was when they ordered all the parts. It does suck, but it's what they gotta do so they don't lose money on the sale. I'm sure there's a little more to it than that, but without access to their entire balance sheet all we can do is trust that they know what they're doing.
Also, if you'd ordered when pre-orders were first offered, the exchange rate was a lot more favourable and you would have saved money. Consider the extra 20GBP a "tax" for waiting so long.
(someone asking for entire balance sheet in the interest of being "open" with information in 3... 2... )