By paying an extra supplement of £7.50 they will allow you another 5kg bringing your total to 20kg.
Also 5kg in your hand luggage.
I have just dropped my daughter off at the airport with overweight bags even after paying the supplement for an extra allowance.
Wait for it,
Daughters case weighed 29kg
Hand luggage 10kg.
Making her 14kg over the allowed limit.

I told her to be prepared to pay quite a lot for being above her allowance.
What do I know,

She went up to this very nice man on the desk,gave him a smile and engaged him in conversation,
she mentioned her case was overweight, so he smiled,tied on the labels,
gave her, her flight tickets and waved her through, no charge.
Now there was a lady on this next desk who my daughters says was weighing everything,cases and hand luggage and then informing the people of the relevant charges they had incurred.
Moral of this for me is,
When I check in,
I am going up to the man on the counter,smile at him,maybe just flutter those eyes of mine that seem to make the Turkish man go weak at the knees.
Engage him in a conversation, maybe about the weather and see how my luck fares.
I have not mentioned the airport or carrier for obviouse reasons.
No case is to be carried over 30kg in weight,this also may vary with different airlines.
Gene