Frustrating, citrange. But the Brittish are taff people, aren't they?
My climate is also problematic. Summer are probably a bit warmer though not always. The last days, we had around -10°C at night after most plants had already budded and some had already gotten leaves. This is not unusual for April. Real growth is only possible from end of April to end of August - a pretty short summer for citrus. September and October can be warm but usually not warm enough for citrus to grow.
My US199 is planted inground in my winter garden (better say green house with natural ground) and grafted on Carrizzo. It grows very well there and is now over 2m high. But no flowers yet. It seems to be slow getting flowers. Many much smaller plants of other varieties have already given me a nice harvest. I also have Sucrena (Succory), an elder of US199. It also seems to be slow flowering, got only two flowers last year while the smaller Vainiglia Sanguigno had already given me 14 fruits from countless flowers. So your absence of fruits seems to have a genetic background, also.