If your card is charged in the local currency e.g. USD or EUR then Starling will convert to GBP using MasterCard exchange rates.
If the transaction processor converts the sum from USD or EUR etc into GBP and charges the card in GBP instead of local currency the conversion will be done by them at a worse rate and be outside the control of Starling.
In theory either in the online ordering system prior to entering card payment details or when entering card payment details it should advise you of any currency convertion and if fully compliant with MasterCard Merchant agreements they should give you the choice to accept or decline any currency conversion.
Normally it is best to decline currency conversion offers and let transaction go thru in the non-GBP currency.
If they fail to notify you of the currency conversion then this is technically grounds for a potential claim back under the card scheme rules.