Re: Risk vs Reward
Why should less favourable compensation terms of a contract between two parties be tax deductible? The risk taken between the parties is by mutual agreement and by choice.
It is fair to ask for more pay when signing up to a contract with less favourable terms (as you mention no holiday, no benefits, short to no notice period and so on) but that is between the contractor and the client and does not require HMRC to subsidise/encourage this via tax policies.
Specific HMRC tax deductions (for business risk for eg ) should result in an increase in tax revenue in the long run which is of benefit to the country (and not one individual).
A contractor wanting more pay in lieu of benefits does not need to be specially tax deductible.
Similarly employers wanting to avoid paye/employer liabilities is not something HMRC should make specially tax deductable to encourage. It is usually one customer, for long durations, distinguishable from a paye only in these liability benefits.
This is very different to the risks taken to start a genuine business, where there is a genuine attempt to gain multiple customers. It is clearly distinct from paye employee beyond just compensation terms.
It is of no economic benefit for HMRC to justify a special tax consideration as there will be almost no distinct growth potential in revenue/employment creation as there is no intent in these cases for the contractor to do so.
Such a deduction will not increase HMRC revenue take over time (by increasing economic activity).
Whether IR35 is appropriate or suitable is a different point.
So no I don't think *any* risk is an automatic entitlement to reduced tax outlay. I haven't seen any of these single customer contractors taking anywhere close to the types of risk a typical business takes.