I've just taken a look at the Parcel2Go website (which is a parcel broker, searching all of the main carriers and offering discounts against booking directly). I've guessed at the dimensions of your package as being 120 x 50 x 20 cm, with a weight of 3kg.
Parcelforce Worldwide's services (when booked through Parcel2Go) start at £12.24. TNT charges from £18.77, UPS from £24.64 and Hermes £29.99. (The other carriers on the Parcel2Go website won't handle the weight/size of such a package).
So it looks like Parcelforce Worldwide might be your best option.
https://www.parcel2go.com
(Note 1: Click on 'Is your parcel longer than 1m?' to access the full size & weight options.
Note 2: Ensure that your read the smaller VAT-inclusive prices, rather than the bold non-VAT ones.
Note 3: The prices shown there only offer minimal compensation in the event of something going wrong but you can increase the compensation limit after you've selected the service you want to use).