I've been learning Chinese off an on for a few years. It will take about 3-5 years of intense study to get to a level of Chinese where you can work in Chinese.
I'm actually interested in moving to China myself, but I'm not finding many offers in my field. Apparently I'm also too old according to this article (31). But it would be only for a few years anyway.
She might find a job in China where she speaks English. For learning Chinese you have to be really motivated.
Since you're coming from the EEA, I'd be hesitant to take up employment in London before the Brexit negotiations are done, mostly because that uncertainty may cause a lot of emotional stress.
If you're optimising for "emotional stress", London still wins over China, for someone who speaks English and not Mandarin, and is comfortable in Western culture.
The government has made pretty strongly phrased guarantees for EU nationals arriving before 29 March 2019. Sure, it's not the best time to plan to put down roots permanently, but nobody who just wants to work for a few years should be discouraged by Brexit at this point.
I am a 29yo European software developer with 4 years of experience working in Switzerland, with both startups and big companies.
My partner can't find a job in our current country but she has offers in China (Guangzhou) and England (London).
What do you think would be more valuable for me career-wise? Does it make sense to invest the energy learning the Chinese language and customs?