British expression
From Downton Abbey:
1. If only to spare my fingers. Is it a common use in daily life? It seems to make no sense for foreign English learner.
2. Many details call into question the case for premeditation. I wonder if this is a completed sentence.
3.Not that I can think of? Or other conjunction follows not, like not when.... How to translate it?

Question

British expression
From Downton Abbey: 
1. If only to spare my fingers. Is it a common use in daily life? It seems to make no sense for foreign English learner.
2. Many details call into question the case for premeditation. I wonder if this is a completed sentence. 
3.Not that I can think of? Or other conjunction follows not, like not when.... How to translate it?

Answer

1. It's not a common sentence but we have other expressions such as 'Work my fingers to the bone,' which is the opposite of 'If only to spare my fingers'
2. Yes it is a complete sentence.
3. I can't help you there as I don't know Mandarin. However if there is a way to negate a sentence in Mandarin, do that.

Answer

There's nothing 'British' about 'if only' It's American English too. It's just that you happened to hear it on a British show. 
It is a little unusual these days, a bit dated.