@PattiPritell
All the advice I've read about finances says spread your money so some in ISAs some in property, some in funds Then if something tanks due to a disaster eg covid when I think most shares fell in value you have other stuff safe eg property going up. Shares have recovered now.
Spreading your money ie "diversification" among different investment types makes sense, but one challenge is that real property is "lumpy": either you own one or you don't. You cannot easily sell half of a property, and I think the OP has to be either 100% property or 0% property.
Also in reading language about ISAs, funds, SIPPs, stocks and shares, etc. it is important to distinguish between containers and contents. Funds and ISAs are both more like types of containers than contents.
An ISA is a type of container that is designed to deliver certain tax benefits: after-tax money goes into it, but earnings within the container are tax free and money can generally be taken out when you need it. A SIPP is a different type of tax-related container: pre-tax money goes into it, but access is very restricted and the money including earnings gets taxed upon withdrawal.
A fund is yet a different type of container that is designed to hold a basket of assets, often stocks or bonds, so that an individual investor can invest more easily and with more diversification than if they bought individual stocks or bonds. A fund can be purchased within an ISA, or a SIPP.
Diversification is worth having in the sense of having different types of contents, so for example some stocks, but not everything in stocks. In normal times you'd also want some bonds, but currently bonds yield next to nothing so it might be better to have stocks and cash and skip the bonds.
Whether that is best done within ISA, SIPP, normal taxable account, or a mix of the three, depends on the person's tax circumstances. But "diversifying" among ISA / SIPP / taxable account does not really make sense, and more importantly, it is possible to have a poorly diversified portfolio while holding all three of those or while holding many different funds. What matters is what's contained inside the funds / ISA / SIPP / taxable account.
Going back to the original question, having investment property in a diversified portfolio, is a good idea in general, but in the OPs case, as
I read it, it's either 100% investment property or a mix of stocks and bonds, but not both, because the OP would have to sell 100% of the property in order to buy any liquid financial investments (which include stocks and bonds).