Hey purplevikki,
Upgrading any flight out of the UK prior to boarding the plane will incur local surcharges and taxes.
Unfortunately the only way around this is to upgrade on board - however this is only eligible for skywards tiers silver & above (I see that you're skywards blue in your profile).
As long as you're willing to pay taxes, your best bet will be to upgrade at check-in if space is available.
Hope this helps.