As has been suggested, Parcel2go is an excellent parcel broker offering discounted rates through a variety of carriers. Even if you were able to use Royal Mail or Parcelforce it often makes sense to go via the Parcel2go website anyway. You then pay less, print your label out at home and drop of the parcel at your local post office for free. (The only thing to watch out for is that the default level of insurance when booking via Parcel2go can sometimes be less than that you get when booking directly with a particular carrier. You can, of course, opt to increase the level of insurance but you need to build that into your calculations when comparing prices).
For carriers other than Royal Mail and Parcelforce, packages booked with Parcel2go are usually taken to a local pick-up point for the relevant courier company but with some carriers offering a 'collect from home' service as well. At the present time, all carriers that Parcel2go offer through their website are providing a 'collect from home' service.