If you need it to be in theory mobile in planning terms then I would strongly recommend getting one which has hit and miss line of boarding at the top of the back wall, as that vastly reduces the risk of high winds (which are becoming more common) making the whole thing take off in gales.
The only people I've seen who do it as standard is Saltire stables, but even if you customise one that isnt designed in that way, it would be well worth it, although it might be worth seeing if saltire do go down your way.
I wouldn't bother with secondhand unless it is close, cheap, a good make and I had a handy OH and transport.
We have one from a local guy in the midland (so sorry no good for you) - happy for anyone to PM me for his web site.
It still looks as good today as it did nearly 6 years ago when I bought it (other than the chew marks where the delightfuls had a try of how it tasted! - but I can't blame him for that) it's had nil maintenance.
Good quality thick presure treated wood is key and I went for metal skids so non of it's wood is actually in contact with the ground - hence no rotting.
It was a good price (£3k ish) for 24 by 12 split into 2 with gates on the front (so effectively two stables), he came with 4x4 and trailer with all the parts on and built it on site in under a day.
I also got one from the north midlands. Fantastic 24x12 split into two we paid around 3k 3 yrs ago. I sold it end if last yr he came back dismantled it and rebuilt for new owner but far but pm if you want details.
The wrong end of the country for you, but might be useful to anyone in the Highlands - Novar Tulloch. Their website appears to be down temporarily, but they're at Evanton, just north of Inverness. We have an 18x12 shelter on skids in a very exposed location and the only time it's ever moved is when we had a gale that gusted to 105mph, when it shifted about 18". Anything up to about 90mph it's fine.
I bought a mobile field shelter from Chart Stables approx 3 years ago. The lady on the phone advised me not to pay extra for metal skids as the wood is good quality and by the time the skids start to rot the rest of it will be too!
The field shelter tows really well, is very robust and I am very pleased with it.
I bought a great field shelter/mobile stable from National stables which was fully pressure treated wood, line kick boards, stable doors etc. It survived the battering my bolshy cob gave it when the hunt used to come past and he would try to kick his way out to join them, not a hint of damage - couldn't rate it highly enough. Had metal skids and stood up to the high winds as I live on a hill and it can be brutal coming up the valley. I'd steer clear of one that has roofing felt nailed onto chipboard roof as the felt comes off so easily and then everything leaks and the roof gets spoiled. I recently sold mine as my boy was pts and luckily it went to a local farmer who could move it in one on bale trailer as dismantling it would have been epic!
We have two. Both were around £1000. The first one is five years old. We had metal skids made for it by a local metalwork firm, and have moved it a few hundred yards to the stable yard onto an existing concrete pad. We had two doors made for it - one a regular top and bottom stable door, the other a floor to ceiling side bit that bolts into the floor and top of doorframe, thus making it a stable if we need it, but still has a door wide enough to roll big bales into (thats what we use it for - haylage store). We had another one made the following year as a straw store, but this time we left it without skids. The whole thing is built on wooden skids anyway. Both have sky lights and are half boarded. They've never moved at all, despite being at the foot of the moors in the Pennines, with a lot of wind.
I'd look for local firms and go and see some of their handywork - ours could introduce you to other people locally who have had their products. Some people came to look at our stables! Some of them are just as good as some of the big name firms but without the glossy brochures and ads, and half the price.
Very happy with ours which is 36' by 12' made up of 2 stables each end with tack room in centre as it is used to house 2 stallions. It is on metal skids and also has light panels in the roof which makes the stables really light inside. We have fitted a solar panel unit to give us electric light for the dark evenings.
The Company Sheds and Stables Direct could not have been more helpful as originally the 2 stables were side by side with the tack room on the end but when we explained we wanted to house stallions they swapped the plan so the tack room was in the centre. All are 12' x 12' so could be made into 3 stables instead with just the kick boarding to add in the tack room and change the door, and delivery was a fortnight after we put the order in. Total price including delivery (seeing they are in Cheshire and we are in Cornwall) was £3,300 including VAT and erection cost was £200 and completed in a day. We did have some horrendous quotes and one of the worst ones was a firm in Devon.