I think "Mail Boxes Etc" is probably our UK equivalent of UPS Stores. Amongst other things, they can do packaging and labelling and act as a broker across the various couriers. I've had good experiences with MBE but I've only used them to ship high value items internationally - I don't know how cost effective they are for other stuff. And obviously, they're still at the mercy of the couriers (though you can at least tell them which ones you want or don't want to use).