I am afraid entering into a marriage of convenience for immigration purposes is itself illegal and you were a party to the illegality.
none of those are good reasons. If you had entered into a pre nuptial or a post nuptial agreement and both had taken independent legal advice, then that would have helped you but the fact that there is none really means that the starting point of 50:50 comes into play.
His conduct may be taken into account by the court if it appears to be inequitable to the court to disregard it and this may lead to him getting say 40% and you 60%, but like I said, the final decision will lie with the court.
Under S. 25 of the Matrimonial Causes Act 1973, the following factors are normally considered:
(b)the financial needs, obligations and responsibilities which each of the parties to the marriage has or is likely to have in the foreseeable future;
(c)the standard of living enjoyed by the family before the breakdown of the marriage;
(d)the age of each party to the marriage and the duration of the marriage;
(e)any physical or mental disability of either of the parties to the marriage;
(f)the contributions which each of the parties has made or is likely in the foreseeable future to make to the welfare of the family, including any contribution by looking after the home or caring for the family;
(g)the conduct of each of the parties, if that conduct is such that it would in the opinion of the court be inequitable to disregard it;
(h)in the case of proceedings for divorce or nullity of marriage, the value to each of the parties to the marriage of any benefit. . . which, by reason of the dissolution or annulment of the marriage, that party will lose the chance of acquiring.
This article may also be useful for you to understand the matter more:
Hope this helps