There is no requirement that the asset should be used

That's exactly right.

There are various defences to this, otherwise everybody working in a company that owns a bizjet would get taxed on 20% of its value! For employees, company rules and an employment-contractual ban is one. For Directors, especially controlling ones, it's harder. It's a lot harder if some private use is in fact desired; then a booking website is a key ingredient, and it gets quite messy.

But, as has already been said, the arrangements used by individual cases are settled in local deals so everyone is going to get a different treatment depending on which Gestapo officer they get.

A lot of people who own a company plane cut a flat rate deal with their local inspector so that e.g. 75% of their flying is on business, and they have to repay the other 25%.

Probably the cleanest way to do a combined business+private situation is:

Buy the plane in person, via Denmark (zero VAT and no need to be VAT registered to reclaim the purchase VAT), own it yourself, and get the company to reimburse you for the appropriate % of the total cost of operating the plane, according to the % of business hours flown. So if e.g. the plane cost £20k for the year to run (incl depreciation, annuals etc) and 75% of airborne time was on business flights, the business can repay you £15k.

You can't invoice the firm for VAT so the firm will not be able to claim back any VAT on that £15k. However, the firm can buy fuel and aircraft parts, pay for maintenance, etc, to the value of that £15k and it can reclaim VAT on all of that.

One needs to be able to support every "business" flight, obviously, with evidence that it was a real customer/supplier visit etc. Just like with a car.

I have the above from an accountant; hope it's right. If so, there doesn't appear to be any way the Revenue can challenge this.

Another advantage of the above is that no rental is involved so it is OK to do it with a private CofA G-reg. Or an N-reg of course.