As a 'qualified professional', I must say that I would rather be guided by a real live accountant, (or a good one, at least), with an intimate knowlede of the business in question, than the forum. This is not to denigrate my fellow contributors, many of whom are clearly of a very high calibre and would be entitled to charge significant fees for the advice they give freely here. Simply to say that, no matter the level of expertise, no matter how well crafted the advice, there's no substitute for "boots on the ground", so to speak.
Like you, my hackles would rise in the presence of an Inspector who bragged that his assessments had forced businesses to close. Perhaps presented in a way to indicate that you'd be better off settling for what might by comparison be much easier to bear? Too much of that kind of talk might make me inclined to speak to the Inspector's manager. It's not acceptable to 'bully' a taxpayer. I should stress that this is NOT a decision to be taken lightly, however. There is a very real risk that this will make matters worse, so you must be sure of your facts, etc.
I don't think your accountant is necessarily a bad adviser. I don't know the size of your business, the amounts at stake, or how much your accountant charges. He or she may simply be exercising restraint because of the relative cost of the assessment as against professional fees in fighting the case.
I wholly agree with R: inferring that the petrol was paid for on private shopping trips simply because supermarket filling stations were used, is a questionable bit of detective work, to say the least.
I feel there is some mileage in resisting the Inspector's assertions. Unfortunately, without knowledge of the case, I cannot advise on when or how to do so. Certainly if he's now put his case in writing, his bolt is shot, to a degree - it would look quite churlish if you disputed this point and he then threatened to open up other aspects - if he had genuine doubts about other matters, they should have been presented and made clear from the beginning.
One final point, picking up on what R said - if this pool car scenario is still going on, then keeping accurate records now, of who uses the cars, when and why, for a period of say a month, might provide an excellent specimen for contending that they are genuine pool cars. It's not as good as records from the period under enquiry, but if you can reasonably demonstrate that your business model/use of the vehicles hasn't substantially changed in the intervening period, then I see no reason why the Inspector shouldn't accept your contention that they were pool cars beforehand as well.