In my opinion, those who have responded up to now are too optimistic. If your wife is not motivated to improve her English now in order to discuss a wider range of topics with greater accuracy and linguistic range, there is a pretty good chance she will sink rather than swim after moving to an English speaking country. By "sink," I mean that she will not be able to develop quickly enough the language skills needed for her to maintain the same kind of socio-economic status she has had in Taiwan. Pretty much any kind of white-collar job back in the UK, like any white-collar job in Taiwan, requires more than just intermediate-level language skills in the local language, and failing to demonstrate those skills when looking for such jobs means she won't get them. This may have a big impact on her desire to assimilate.
The assumption that moving to a country where English is spoken will sort out a non-native English speaker's language problems is frequently incorrect. Does she get along really well with your family? What about your friends? Does she have a career in Taiwan that would allow her to slot right into similar career in the UK, or are her qualifications and experience such that she would have to get new qualifications and job skills while also having to improve her English? Does she have a range of personal interests (e.g., sports, hobbies) that she can pursue in the UK with English speakers so as to develop her own network in a way that is not dependent on you? Or do you think her personality makes her more likely to settle into a social life filled with other Chinese speakers? Also note that she is not getting younger, and a lot of her problems have probably fossilized. Statements about it being easier to learn a language when younger are made to the point of being cliche, but suffice it to say that your wife probably won't improve at the same pace in say her 30s in the UK as she did when she was younger in Australia-unless she is really motivated to do it.
It is very difficult for one spouse to be the other's language teacher if the one doing the learning is not really motivated and open to feedback. Your wife may unconsciously assume that since the two of you have no major communication problems in English, then the need to improve is not great. The issue here is that people learn to understand the idiosyncracies of a non-native spouses speech, so instances of the kind breakdown in communication that would highlight a need to improve are not so frequent. She may feel that your feedback is just part of the typical nitpicking that some couples do, especially if you are the only person giving such feedback. Strangers or friends are usually too friendly to seek clarification of meaning, so interactions with them are less likely to highlight a need to improve accuracy, fluency and range unless the learner is motivated to the degree of pressing forward until misunderstandings become obvious.
I don't mean to imply that your kind of situation is hopeless. I just feel it is important for people in this kind of situation to recognize the issue for what it is rather than draw convenient conclusions like "once we move back to the UK, this will sort itself out." If your wife isn't motivated, you could have a real problem on your hands. Do the two of you visit the UK often? Perhaps more enjoyable visits or more frequent interaction with English speakers in Taiwan would help improve her interest in learning.