It looks like you've already settled on Collect Plus (which I'm happy to continue recommending) but I'm amazed that nobody here has suggested going through a parcel broker. Doing so not only gets you quotes from lots of different services but can also get you discounts on those that you've tried directly. (In particular, you can get far lower prices with ParcelForce by paying online through a brokerage site and then handing over your parcel, without further payment, at your local post office).
https://www.parcel2go.com/