OP I agree with other posters that the problem seems to be your husband rather than Islam. Has he based his decision solely on the opinion of this one Imam? As a person who was born muslim I wouldnt advise him to only seek religious counsel from one source. Despite the popular media portrayal that there is only one version of Islam, there are in fact lots of differences of opinion on many issues within the Islamic faith based on how religious texts are interpreted and these differences have traditionally been allowed. There is not necessarily only one right way to do things. It is not in your husband's interest to split up his family (hopefully) so he should get lots of different opinions preferably from muslim scholars and muftis on your situation before making such drastic choices.
An imam is used commonly by muslims as a general term for someone who leads the congregational prayers (even a child can be the imam if he knows how to pray) and unfortunately not all imams are equally qualified to give religious verdicts in terms of being classically trained in Islamic sciences. Also if the imam is not originally from the UK (i.e. born and brought up abroad) his interpretation may be influenced by his own cultural background. There is a very strong argument that muslims should seek out religious verdicts from scholars who live in or have lived in the same society as them as they will have a better understanding of their societal norms, problems etc. I would advise your husband to get opinions from scholars/imams that have been brought up in the Western world at least or even better are converts themselves. There are quite a few I could recommend and I can pm you there names if you like.
If your husband tries to pressure you to convert I would remind him of this verse in the Quran.
'Let there be no compulsion in religion' Holy Quran: Chapter 2, verse 256
Also the following verse tells muslims to accept that not everyone will necessarily believe in Islam and we should not compel them:
“And had your Lord willed, those on earth would have believed, all of them together. So, will you then compel mankind, until they become believers?” Holy Quran: Chapter 10, verse 99
I would ask your husband does he want you to convert for your benefit or for his? If it is for your own benefit then the choice has to come from you, without pressure being exerted on you. If it is for his benefit then you can superficially convert but your conversion will not be accepted by God anyway if it is not based on real faith. In Islam we have a principle that 'Actions are judged by their intentions' so if your intention in converting is to please your husband and not because you genuinely believe then God knows what is in the hearts. If you really want to remain married to him (I havent read your other thread but from what others have posted here it appears there are some red flags) I would say that you are happy to support him in his new faith and willing to try to understand his faith more in order to do that but you can not guarantee that will lead you to want to adopt his faith.
I dont think your situation is as hopeless as you fear. If you follow a very literal interpretation of the text then yes it only mentions that it is permissible to marry women from the other Abrahamic faiths (namely jews and christian). However if you interpret it by trying to understand the underlying reasons for that you will see that the Quran allows that because they are monotheistic faiths. So despite the differences between the three Abrahamic religions the fact that they share the same core belief in one god means there is a compatibility there which is not shared with polytheistic religions. For example in general christians and jews would not have idols in their homes or engage in other activities that may cause you to compromise that core belief in one god. I hope that makes sense. Sikhism is not mentioned because it is a much younger religion then Islam, however you would have to look at the teachings of sikhism to see whether it fell in to the monotheistic or polytheistic category. If it is monotheistic there may be more flexibility but you would need to consult a muslim scholar on that.
Also even if you do follow that very literal interpretation of the text he should give you time to research for yourself and decide before he starts issuing ultimatums. It is not as black and white when your relationship began before your husband became a muslim and any imam should be able to recognise that and deal with you sensitively. The daughter of the Prophet Muhammed PBUH was married to a polytheist for many years even after she became a muslim. She had got married to him before her father started his mission as a prophet and began preaching Islam. They did eventually divorce but the Prophet Muhammed PBUH did not force her to leave her husband straightaway and he was sensitive to his daughter's emotions towards him even after she left her husband. Her husband did convert to Islam eventually but this was after they had been separated for 6 years and he then asked her to remarry him. You can read about their story below.
muslimahatpeace.wordpress.com/2011/10/22/the-beautiful-poignant-love-story-of-zainab-bint-muhammad-and-abu-al-aas-ibn-rabee-ra/
Finally (I know this is going to be a massively long post but bear with me) I would let your husband know of another hadith which a very important islamic principle is based upon. The hadith is:
Abu Musa Al-Ashari reported: Whenever the Messenger of Allah, peace and blessings be upon him, would send his companions on a mission, he would say, “Give glad tidings, and do not scare people away. Make things easy, and do not make things difficult.”
The interpretation of this is that you should not go to extremes in religious matters if there is an easier approach. If he can find a religious opinion that would enable you to stay together then he should follow that one as that would make life easier for you, your children and him.
Hope that helps.