Financial retirement planning

Nothing comes close to an approved retirement savings plan (pension) to be honest

Well, you may be paying it off early for when you're retired, and no longer have the income to service a mortgage. Or, you may plan to sell it on retirement, trade down to a smaller or cheaper house, and liberate some capital to supplement your pension.

But you're right. You do need to have an exit strategy here. The OP asks about the most tax-efficient ways of planning for retirement, but tax efficiency can't be the dominant factor. In planning for retirement, the most important question is, what will the OP's needs in retirement be? First and foremost, he needs to choose an investment mechanism which will meet those needs; tax efficiency is very much a secondary consideration to that. So it matters whether he plans to pay off his mortgage out of income before retirement, sell the house and pay off the mortgage that way, or continue to service the mortgage during retirement.

The advantage of putting money into your mortgage is that it earns a tax-free rate of return. You might expect a higher return on your equity investments, but that will be taxed. So you need to ask yourself if you expect a return on your equity investments that is so much higher that, even after paying tax, you'll still be ahead.

The other issue, as already pointed out by listermint, is that investing in reducing your mortgage is relatively illiquid; how are you going to realise your investment without selling the house? While you contemplate that you might sell the house at some point, you presumably don't want to have to sell it when you otherwise might not.

Which makes the point that you have a focus on tax efficiency which may not be appropriate. If your sole concern is tax efficiency, then investing in reducing your mortgage is more tax-efficient than investing in, e.g., an ETF. But you might have good reasons for not doing that.

If tax efficiency is your dominant consideration then, as I and others have pointed out, pension funds and the like are easily the most tax-efficient vehicle for retirement saving. But you are looking for something other than pension funds; this seems odd, if you attribute a high value to tax efficiency.

So, it might help if you could tell us why you are averse to using pension funds for the purpose for which they were, after all, devised?

No offence, but while you express a concern about tax efficiency, when faced with actual choices you have twice selected against the more tax efficient option — once when you decide not to invest in pension funds, and once when you decide not to invest in mortgage reduction. In each case that choice must be driven by something that you value more than tax efficiency. You're not telling us what that is, and maybe you're not recognising yourself what it is. So that might be worth unpicking a bit.

The only other options involve investing in undiversified investments. There's usually some scheme or other for film or student accomodation investments.