I've always used
www.senditnow.com for heavy or bulky parcels.
Senditnow use Parcelforce to do the deliveries and have always been spot on for me.
Max. allowed weight is 30kg, just stick it in a strong cardboard box and it should be fine.
The best bit is the price. £10.99 for guaranteed 2 day delivery, £11.99 for next day or £13.99 for next day by midday. Compare that against using Parcelforce directly!