Legally - not a problem, it comes under EU regulation 1/2005 for you in Wales this is enacted under the Welfare of Animals (Transport) (Wales) Order 2006 - English Equiv The Welfare of Animals (Transport) (England) Order 2006 and another equiv for scotland.
Dog crates are ideal for moving them.
Presuming your journey is under 65km, then
You would need to protect from inclement weather - Chapter II 1.1a means of transport shall "protect the animals from inclement weather, extreme temperatures and adverse changes in climatic conditions" - in your case I would watch for drafts if it an open crate - you might want to put some plastic sheet/tarp round to make it more windproof and protect from rain if needed. For instance 10 miles in cold wet conditions at eight weeks is not good for them, never mind the regs!
You will need to provide bedding - Chapter II Section 1.5 "Piglets of less than 10 kgs shall be provided with appropriate bedding material or equivalent material which guarantees their comfort appropriate to the species, the number of animals being transported, the journey time, and the weather. This material has to ensure adequate absorption of urine and faeces." - so some straw in the bottom.
Technically you need to indicate the presence of animals Chapter II 2.1 "Vehicles in which animals are transported shall be clearly and visibly marked indicating the presence of live animals, except when the animals are transported in containers marked in accordance with paragraph 5.1.
Para 5.1 saying "Containers in which animals are transported shall be clearly and visibly marked, indicating the presence of live animals and with a sign indicating the top of the container"
You should make sure the container cannot move around Chapter II 5.2 "During transport and handling, containers shall always be kept upright and severe jolts or shaking shall be minimised. Containers shall be secured so as to prevent displacement due to the movement of the means of transport."
On size of container Chapter VII sections D "All pigs must at least be able to lie down and stand up in their natural position."
An of course means of transport (the crate, and in your case the back of the pickup) must be "cleaned and disinfected" Chapter II 1.1d
All of above is really just good practise, but the EU has a reg for it !!