Best citrus trees for growing indoors?

They will all do better if outdoors in the summer, but I do have some I keep in the house, most using lights but not all, some just near a window, citrus dont actually need that much light

Most productive and easy to grow is the Calamondin, thhey are often sold as small oranges....but are more of a lime, used like lemon on food like chicken and fish while still green just as they start to turn orange.
Calamondins give lots of fruit as they are small fruit, they dont mind dry air as much so better in the house.

Lemons are most useful of the larger size fruit citrus trees, a bit more tricky to look after but not much, they also carry more fruit, more often than an orange tree

Real orange trees are a waste of time in the UK, they are large fruit so a small tree wont have more than a few every 18 months or so. They need very high temperatures to get sweetness in the summer and then cold to ripen , get flavour and turn orange in the winter,
Limes are very picky to look after, like it very warm and damp

Any decent tree should flower / fruit the same year as you get it, but many people sell small rooted cuttings on places like ebay, they wont....

the smaller / younger the tree the harder to look after it

Usually round this time of year Aldi have calamondin, lemons etc in quite good small trees for about £10 to £15 ,

for a wider range of varieties or if thinking about paying any more than that I would go to a specialist supplier such as `the Citrus Center` or I have had some very good large lemon and mandarin trees from `seagrave nurseries`

Cheers starloc!

They will literally be on the windowsill (no space off the sill nearby), so it's only really the mini varieties I'm interested in. I've probably only got about 12"-14" between glass and curtain. I'll definitely check out the calamondin, and I'll keep my eye out at Aldi Point noted on the young trees too.

From what you've said lemons sound like an OK challenge if small enough, but lime may be a bit too finnicky. Although if it was cheap enough, and there was enough space on the sill, I'd prob still be daft enough to try it