They're still young enough to learn. OPOL (one parent, one language - i.e. your DH would speak to them exclusively in English and you exclusively in your own language) is the gold standard, but if you can't manage that then anything you can do helps. If you can support it with tv, books, music, and any other resources in the minority language (the minority one being the one which isn't spoken around them in school, shops etc., even if it's not a minority language as such) that would vastly help.
A lot depends on the attitude of the other parent. In families where OPOL works really successfully, both parents are supportive. Often the other parent will make efforts to learn the other's language, perhaps alongside the kids, but at the very least they need to be accepting of it and not dismissive just because they can't understand it.
Depending on how common the language is in the UK, you may well find children's groups for speakers of that language, possibly Saturday classes for when they're older (helps with the problem that a lot of kids brought up like that have good oral but very poor written and reading skills in the parent's language - it's called 'kitchen language' because you tend to have a lot of vocabulary associated with the home and not much wider).
Personally (as an MFL teacher), I wouldn't mix and match with English if you can help it, even as a transition. Make the decision with your DH as to what will work, but if possible what you should be going for is zero English with dc. Make it clear what you're saying by using pictures, hand gestures, pointing etc.
Please don't think it's OPOL or nothing. OPOL is the best, but they can still pick up language with lesser input.
Don't expect them to speak back to you straight away. Second language acquisition, when done properly (and not forced like it is in schools) should work like first language acquistion. That means a long time absorbing language before producing any (think of all the months/years a baby spends listening before they make any coherent speech). So don't worry if they don't want to repeat things you say. You just chatter away and let them listen.
If the older one is reluctant (you will almost certainly get whinging along the lines of 'I don't understand' / 'I don't speak X' for quite a long time), the best thing long term is to stand firm and not resort to English. Make sure your meaning is clear if it's anything like an instruction that s/he actually needs to respond to. A picture of a toothbrush and miming brushing teeth alongside saying 'go and brush your teeth'. But no English. When the language has been established, you can translate words either way when a new word comes up, but it should be avoided as much as possible.
It may help to make favourite TV shows, films etc. only available in the target language for a while until you've properly got into it. That doesn't mean no tv in English forever, but just until the second language is established.