The English climate with its long, cool, damp winters is completely unsuited to growing citrus outside. Only the most favoured and sheltered coastal locations, or the centres of our largest cities, provide possibilities for successful growth. The county of Buckinghamshire, where I live, is certainly not a favoured location. We can regularly have frosts from late September to early May and in recent years minimum temperatures have been down to -12C. No true citrus survives for long.
I have thought for many years that Poncirus X Citrus hybrids should be viable and be capable of producing highly ornamental fruit in the UK. Although I have searched widely, I have never found such a tree. I have heard from Agrumes Baches that they once received seeds from a Morton citrange said to be growing in the London area - but I have failed to locate it. However, in late 2010, I received news that there is a fine fruiting specimen of Citrumelo growing against the wall of a private house in the city of Nottingham. See The Citrumelo of Nottingham.
Finally, in late 2012, I was amazed to discover one of my own outdoor trees had produced fruit. I still cannot understand how I saw neither flowers nor developing fruit. Yet one December morning a group of eight fully-formed yellow fruit were glowing at me in the winter sun  Although I had always thought this tree was a seedling citrange, I am now convinced it is actually a seedling citrumelo - rather like the Nottingham specimen.
This tree is really more like a rounded shrub about 2m (6ft) tall and wide. It is growing close to a rhododendron, and the fruits developed quite low down and hidden by the rhododendron leaves.
Although I had always believed I had planted out a Troyer citrange, the fruits' yellow colour and relatively large size have convinced me this is a citrumelo. In June 2000 I was sent seeds of Citrumelo 4475 - also known as 'Swingle' - by INRA Corsica. I think the plant originated from these seeds.
Yellow peel and flesh.  6.5cms diameter, 7.0cms height.The cut fruit shows a very thick pith. This does sometimes happen when growing citrus in cool climates. Seeds were poorly formed, but had dark brown tips. The taste was completely awful! To me, much worse than my poncirus fruits. I was very cautious not to try too much, but the taste was so acrid I had to wash my mouth out several times to get rid of it. Definitely for ornamental use only.

Page created 22nd February 2013, updated 5th November 2013
Poncirus trifoliata X Citrus paradisi
(Japanese Bitter Orange crossed with Grapefruit)
Hardy to at least -12C
In November 2013, I was sent photos from a grower in Petworth, West Sussex in southern England. He said
Amanda Dennis (from The Citrus Centre nursery) tells me it's a swingle or carrizo - I think I must have planted it out after the budding/graft failed on one of their plants and thinking it was a poncirus. It seems pretty hardy and has fruited for the last three years though the inside is mostly pith.
Almost certainly a Swingle citrumelo, as fruits are too large for a citrange, this is now the third fruiting UK specimen I am aware of. Perhaps trimmed slightly into a rather neater shape than mine, the tree is otherwise very similar to the one in my own garden. Fruits also look the same at this time of year, not yet completely yellow.