For the UK, we use both Royal Mail and DPD.
Across the UK we offer Royal Mail Tracked 48 as our free standard shipping option, but also have a paid express option. For the majority of the UK this express option is with DPD, and this is a Next Day service if ordered before noon. However, if you have a postcode in any of the following areas your paid express option will be Royal Mail Tracked 24: Northern Ireland, Scottish Islands, Scottish Highlands, Channel Isles, Isle of Man, Isle of Wight or Scilly Isles.
All three services are tracked, and DPD is both tracked and signed for.
For the US, we use DHL. This is a tracked and signed for service.
For everywhere else, we use Royal Mail International Tracked and Signed. This is a tracked and signed for service.