If you simply want one tree, go with one that is self-fertile such as Victoria. Even self-fertile trees have greater yield when cross pollinated. Late Transparent is good if you are prone to late frosts, but again consider planting more than 1 for greater yield. Fan trained gives more succulent fruit.

Probably one on Pixy rootstock, which is the most dwarfing rootstock of all, and make sure the variety you choose is self fertile if you're only having one. Info below on greengage, including growing in pots and how they need to be trained/pruned

http://www.pots2plots.com/Fruit/Growing%20Greengage.htm

Some info on specific varieties here - these people are quite helpful if you call them

http://www.orangepippintrees.co.uk/plum-trees/old-greengage

This might be useful too.
http://www.independent.co.uk/property/gardening/plum-job-a-juicy-guide-to-greengages-and-plums-2335370.html

(You most likely know that Victoria isn't classified as a greengage in the UK)

Victoria is the nation's favorite and most recognized plum widely used for puddings, jams & pies. If you don't like it, you can also consider Denniston's Superb Gage or Early Transparent Green Gage, two excellent varieties that are also self fertile. No need to be concerned about grafting since they are smallish trees suitable for large containers.

To the best of my knowledge Victoria plum is not a greengage Bathgate which is what john boy wants...

That's why I suggested Denniston's Superb Gage or Early Transparent Gage as well if you didn't like Victoria. They're all very good.

I would go for something like Cambridge Green Gage (AGM) on VVA1 or Pixy rootstock. Can be bough his autumn as a bare root from a reliable nursery, Blackmoor would be my selection. But, much better to grow in the ground for best results.

Personally, I like Victoria for flavor

lol! Here in the States, the tree wholesalers just label them "Greengage", and leave it at that. I always thought that there was more to the story than that. I'm going to ask my vendor rep which specific variety I have been selling all these years! I probably won't get an an intelligible answer--to misquote a famous TV doctor, "**** it, Jim! I'm a salesman, not a botanist!"

As Mg says, 'Victoria' is a plum, not a greengage. Do an internet search to learn the difference between the two. :)

Tug, are you saying that, in the States, any plum tree is sold as a greengage? If you are, Churchill's comment springs to mind yet again - ("two countries divided by a common language...") I have frequently noticed that most people in America tend not to use or know botanical names, rechristening plants in such a way that the rest of the world cannot/does not recognise (Linnaeus and his system of botanical nomenclature, apparently, can go hang himself) I guess this is yet another example in a similar vein! For instance, the business with Vinca drives me bonkers, seems the name vinca in the states is used for both Catharansus and true Vinca... how does anyone know which plant they mean there then? I'm mystified...

Thank you Tug that explains a lot... I'm somewhat astounded that plums get labelled as greengages in the US but at least it solves the riddle of why Bathgate was suggesting a Victoria Plum tree. In my experience you couldn't grow one in a container anyway!

A little research goes a long way.

Not just any plum, but just the varieties of European plum with fruit that is green when ripe. And, like I said, the growers seem to imply that there is only one variety with green fruit. The others are labeled as plums, with rather generic variety names tacked on, such as 'Damson', and 'French Prune', and so on. The Oriental plums (Prunus salicina) are usually labelled more specifically as to variety.

Plums don't get labelled as greengages here in the US Mg and not all gages are green. Greengages are a specific European Cultivar.

Bathgate read Tugs comment as he states that plums do get labelled as greengages! I have never said all greengages were green...

We grow all kinds of plums in the US, including greengages, yellow & purple/blue. Greengages aren't very popular however.

The green ones used to be more popular here in the desert, because the birds didn't realize that they were ripe until the humans had a chance to harvest. Also, whatever variety we were selling was relatively low chill for an European plum, which is a big help in our near subtropical climate. Sales of all deciduous fruit have been down the past decade, since there are fewer true gardeners in our area, now.