Suggestions for my own sepp holzer style lemon tree experiment

Ok, I am making a hugel berm sun scoop type thingy on my allotment. The aim is to show/discover what can be grown in the temperate climate of rainy damp middle england. What I can't decide though is what plant would be best to grow to show off the capabilities of such a technique. I'd love to do a lemon tree experiment but because it's not my land and only an allotment and also due to the lack of machinery and materials, it's only going to be 3 or 4 feet high so would best suit some kind of shrub. So does anyone have any suggestions? I would like it to be something that is recognizable and normally associated with warmer climates.

Four feet might be tall enough for a little lemon tree, or if you want something somewhat more hardy, I think the Kumquat is more hardy than the Lemon.

I've been researching a bit about citrus to grow in our greenhouses, which go a couple degrees below freezing on January nights. What I've read is that the Meyer lemon is more cold hardy than most or all other (tasty) citrus, and it's naturally dwarfed. AND it can be very sweet and tasty, though that depends on conditions and sun.

There's a type of mandarin orange called "ARCTIC FROST HARDY SATSUMA" that is said to be hardy to 10F. Something like that would likely have a lot more impact as they are more agreeable in flavor to a broader range of people than a lemon. I think that is a little more cold hardy than the Meyer lemon as well. I don't know how available they are overseas, but there are a couple suppliers here in the states (Tyler, Bob Wells Nursery in TX is one of them). They're not cheap, but for a single planting the might be worthwhile.