Yes, you're right, Loomo. It's quite common for investors to invest in a mixture of debt and equity, but there are complex rules around what is called 'thin capitalisation', i.e. having too high a debt:equity ratio. In many cases HMRC will disqualify for tax purposes most (if not all) of the club's interest expense as a consequence. There are other measures they can take too, particularly if they belief the interest rate is not akin to arm's length.
From the investor's perspective it is useful to invest in this way, as it can be a good way of recovering some of your capital early, a good way of receiving a priority pseudo-dividend ahead of ordinary shareholders, and a good way of realising tax losses should the debt get written off (as appears to be the cae in this instance).
By Christ this stuff is boring...sorry.