Up in dark and cool England, we have a short growing season where we only get a breba crop off any figs unless we grow them in a greenhouse. 

That's fine, however limited space and budget means that's not really within my reach right now :-(

There is a cheap n' cheerful budget option, however, in polythene/PVC greenhouses (something like http://www.amazon.co.uk/Compact-Walk-In-Greenhouse-Shelves-Heavy/dp/B003JTF9B4/ref=sr_1_6?ie=UTF8&qid=1399492291&sr=8-6&keywords=greenhouse or http://www.amazon.co.uk/GARDEN-OUTDOOR-GREENHOUSE-PLANT-SHELVES/dp/B002UOUZQS/ref=cm_cr_pr_sims_t#productDescription if you've not seen them). I'm a little suspicious that the green colour of a lot of the plastic will take out a fair bit of the sun and so the benefit would not be so great. They do have a reputation for being a bit flimsy (hence the blow-away nickname), but at the price I could get one a season and still not have paid for a whole greenhouse in a decade!

I think these designs at around 6'5" high x 4'9" wide should be good for 1 tree, especially as mine are still quite young (2-3yrs).

Has anyone in a short growing season area tried these, and did they have any effect on ripening a main crop? Our climate is supposedly similar to the pacific NW with drier summers and wetter winters.

got me a 10 foot by 20 foot one off of ebay for under 300.00 bucks U.S. 

I had one very similar. They need to be tied down or weighted down. Mine blew over more times than I could count. I could not honestly recommend them.

I think you'd be better off building a PVC green house for a couple hundred bucks. These light weight pieces of junk aren't worth putting up over and over again.

the one i got has a really good frame. the one i had before not so much. mine is tied down with a few rachet tie down straps across the width. the straps are conected to 24 inch rebar that i pounded in the ground at a 45 away from the green house. pounded em in all but a couple inches. here in the midwest we get some nasty storms with high winds around 80 mph with gust over 100. the green house has stood just fine through all the weather for the past 3 years. 

I'm not too concerned with it being blown away as it'll be up in a sheltered spot next to a wall and quite close to a fence (this also means it's a very limited space - probably 3'6 max width which would make a more permanent structure difficult and not really cost-effective - £200 to coddle one fig tree is a bit more than I'm looking to pay!)

I've had a smaller gro-bag version before which, once I managed to tie it to the wall with some vine eyes and twine managed to stay put all summer, though as it had a flat roof it managed to get a big puddle on the top which pulled up the plastic round the edges.

What I'm really looking for is whether they'll do any good helping my figs give me a main crop or whether their ripening abilty is much poorer than a proper greenhouse in which case they'll just have to cope with the cold and rain like the rest of the garden!

If it's really that small could you find a used glass door and build a small shed around it?

I suggest passing. They're garbage. I have the one in the first link. Got it 6 months ago. Going to throw it out. Horrible design and extremely cheap material. Zippers were the first to go.

I got one like it with a clear plastic cover from Big Lots, a floor model on clearance. Really cheap... in more ways than one. I had it up against the bldg. and it blew away anyway. When I got it anchored good it started coming apart whenever the wind rattled it, the tubes would pull out of the corner connectors and the shelves would fall down. Then it collapsed under the weight of the pots I put in it. I didn't think there was that much weight but those plastic corners are very flimsy and they broke out so they couldn't be fixed. I had to pin the door closed because the zippers stripped out in very short order. I'm glad I didn't pay much for it because it sure wasn't worth much.