Are you really sure that your father-in-law will try to screw you over? Isn't he more interested in ensuring that his son and your family as a whole have as minimally as messy a divorce as possible. If this is his job, he won't want his family, even those he doesn't like, to have to go through the stress, upset and waste of money that going through lawyers entails. And I say this as a professional working in this field.
He could be a useful source of advice for you both for free. If you can be amicable with your husband over the divorce and the financial arrangements, his father might be able to help you both reach an agreement. The English / Welsh rules are quite simple for most people and his reputation will be on the line:
(a) compute the value of the assets (including pensions) generated by the marriage and any pre-marital seamless cohabitation and add in the family home, no matter who contributed what to that (assuming the marriage is over 6 years or so and the house is not held by a formal trust or some other structure) - you do this by providing each other with all your financial details of what you each have - bank accounts, credit card debts, savings, investments, and property interests, and pension cash equivalent values;
(b) divide by two, add mortgage capacities of each party, and adjust the assets together with exercise of mortgage capacity to meet needs of each party;
(c) invade non-matrimonial property (pre- or post- marriage acquired, inheritance or gift) onlt to meet need.
(d) spousal maintenance to allow the financially weaker party some time to become financially independent, if necessary.
(c) child maintenance in accordance with CMS formula (generally).
Once you have a proposed arrangement between you go and see a one couple / one lawyer service, or otherwise you take some independent legal advice then to see if the proposal you have reached is reasonable. If it is, then you can submit your statement of those arrangements to the court for approval of an order to settle everything.
In short, if you can, use the assets that you have by way of your father-in-law's knowledge and save tens of thousands of pounds.
If his father won't help you both out, then encourage your ex husband to use him as his advice source. It will be cheaper overall, assuming his father won't charge his son for it, and there'll be more to divide between you.