Post separation assets and liabilities can be ring-fenced although this is not always the case.
If a liability was incurred necessarily this can be taken into account in the matrimonial finances. As an example, if husband left the wife and children and didn''t pay towards the house and children - it would be reasonable for the wife to incur debt to maintain the property and provide food, heat and clothing. It would also be reasonable to expect the debt to be paid from the joint matrimonial pot.
However, if the husband had entered into a rental agreement for a 4 bedroom house in Mayfair where he lived on his own, it would be unreasonable to expect the wife to subsidise this extravagant level of spending.
In the case of purchasing a car, this is down to necessity and reasonableness. If you do 2000 miles per week a leased car makes sense although an Aston Martin would be an unreasonable expense. If you already had a car which would have been fine in the short term, you might be criticised for making unilateral decisions.