Grapes are great to grow in a greenhouse that size. Big luscious greenhouse types hanging down. wonderful. I know it is not that "exotic", depending on ones view but melons are much more reliable under cover. Also not a fruit but what about sweet potatoes if you have room for them. 
For truly exotic fruit my suggestions are these five in no particular order:
Chilean guava (Ugni molinae)
Pineapple Guava (Feijoa sellowiana)
Strawberry guava (Psidium cattleianum var montana)
Cape Gooseberry
Pepino (Solanum muricatum) I have a couple of these in their second year. One has just set fruit and should ripen in early winter - their usual time. Quite cold tolerant, to -2 perhaps, I grow them in a just frost free greenhouse.
I have found that both oca and yacon do really well outside on the open allotment so there is probably no real need to grow them undercover
Yes to all - but I would say the pineapple guava is also quite happy outside - maybe just not in the frozen north. They can also get quite large for a bigger crop - but they use them for hedging in California so there's no problem with pruning. Fantastic edible flowers like marshmallow but the fruit have a whiff of hospital corridor - iodine and TCP.
Apricots are also good outside on a sunny wall in most of this island - I would go for Lord Napier nectarine instead - wonderfully fresh pale green flesh - like a white peach that's been sent to heaven (and been shaved).
You really should check out the Custard Banana (Asimina at agroforestry.co.uk) - they are the closest thing to a mango that will grow over here - they laugh at our winters (they grow wild up the eastern USA to the Canadian border) but they do benefit from having the summer under glass. Unfortunately they hate transplanting and aren't really suited to pots - it's easier to move the greenhouse over something that appreciates shelter in winter - eg. as it happens there's a nice banana shaped passionfruit with a taste like mandarin - that has the opposite requirement.
I now much prefer the fully annual ground cherry (outside) to the closely related partly perennial cape gooseberry under glass - the latter are just too aromatic - they give me a petrol headache.
The new hybrid grapes are so delicious and early (x indigenous N.American species) - I would only bother growing grapes under glass if they were true Muscats.
As to melons - in return for a LOT of effort and a LOT of expenditure you have to be an absolute expert to beat or even equal the best types available in the shops - mainly because they DO ripen during transit. It's much better and probably cheaper to holiday in the more remote parts of the Med. and taste them at their best from local markets and roving truck sales.
There are some really nice kumquat crosses that would enjoy a cold house - Kumandarin , Eustis limequat, Clemkumquat. The Rangpur lime is also pretty tough. Don't try the Tahiti lime in a cold house (despite the adverts), though it is fine in a conservatory at 4C. Meyers lemon is tougher than true lemons and much sweeter - it's a mandarin cross.
I've left my personal favourite to the last - epi hybrid cacti are jungle cacti and produce fruit much tastier than dragon fruit - if you hand pollinate them. The 20-30cm flowers are spectacular - though the only ones that will stand -5C are the "ackermaniis" and their fruit is a bit too "boiled sweet" for me. "Samite" is a good fruiting variety but you'll need a pollinator - almost any of the others will do though it's better to have five or more different ones for other flavours - you can never tell which ones will flower alongside "Samite" next year.
My only quibble is with sweet potatoes - every time I have had a "healthy" orange one I've always thought I'd rather have had a (healthier) carrot. I don't like much stuff that cooks to mush - I always eat my pumpkin grated into coleslaw. The white SPs are OK - possibly a bit nicer than normal potatoes but not really worth the effort.
Cheers.
