Parcel2Go, Interparcel et al are all "middlemen". Go with them and it's up to you to decide which courier co (ie Fedex, DHL, TNT, City Link etc.) they use dependent on price, service levels etc. It's cheaper than going directly with the courier company if you're just an infrequent user.
Personally I've always used Parcel2Go and used the DHL Home Economy 48/72 hrs service. Good prices, track and traceable, and in every instance has been delivered next day. Also, so far touch wood everything has arrived in one piece.
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