Start by using a parcel broker's website to see what's available to you. Parcel2Go is probably the most well-known site (and the one I use):
https://www.parcel2go.com
The prices you'll find on a broker's website are cheaper than those you'll find by going directly to a carrier but you should note that the basic level of insurance cover might be lower. (You can always increase the level of cover but you need to take that into account when comparing prices).
The service I use most frequently (either directly or through Parcel2Go) is CollectPlus, simply because they've got a drop-off point at a local Co-op near to me. I've never had a problem with them:
collectplus.co.uk/