IR35 is applied per contract. So HMRC would come after him for BT work and anywhere else he had been working for a period of time.

It is up to the client to determine the status (so all companies defer to saying the 'contractor' is inside IR35.)

If inside IR35 and for example you earn 100k a year - from memory your are allowed 5k in expenses - the rest is treated as taxable(income tax plus national insurance) the bbc also have to pay employers NI contributions.

Lineker lost his case and had to pay 4.6 million - his appeal is up shortly (I hear May - but seen it confirmed)