Reuter Brooks is based in SE1, just off Druid Street. They're very good and cheap too if you have an account with them. Not sure about cash/card prices, but sure they're reasonable too. No connection etc.
Go to Parcel Monkey.co.uk
Their own service, parcel monkey , sent via DX is very reliable and very cost effective ( and you don't need a label as the courier arrives with a large bag into which you place your parcel) . I have no connection with.. but use a number of couriers regularly and this is ideal for not over large parcels.