Admittedly, I have no first hand experience with SQ, but I would expect any SQ ticket office or call center to be able to reprice and charge your credit card in the currency you prefer (using the much more generous exchange rate in the airline system when converting from NOK til GBP rather than the exchange rates of your CC which easily comes with a 2-4% mark-up). Obviously, you will have to pay applicable fees to SQ for making the booking via phone, but sometime you are able to get the fee waived by stating that the itinerary was not possible to book online (or state it as it is, you want to pay in GBP rather than NOK). Good luck!