The problem is £20k will only buy one like you have linked, which doesn't have CIC nav, the DCT gearbox or EDC. To get these at a reasonable mileage then you get up to and past the £30k mark rather quickly. Market value doesn't always equal fair value, just look at E92 335i's, last I checked for a good spec LCI they were nudging £20k, now even though the car shouldn't be worth that, the market determines that's what they exchange hands for, which will also work in your favor when selling (although probably not for a 335i, see my point about F30 335i below.)
The problem with an F30 335i or M235i is that they don't feel special enough, nor do they have a V8. They're also at the £30k price point quite early into their production where an LCI will most likely happen during my potential 3 years of ownership, pushing values down even further, and no matter how amazing an example I buy, why will people pay top money for a pre-LCI large petrol engined normal 3 series when F30 M3s will be depreciating towards an achievable value?
I get the whole argument against spending lots on an older generation car, however it's not like you take the entire £30k and throw it down the drain, the (albeit heavily depreciating) asset still has value, which will still have a premium over the 2008 poverty spec M3s in a few years time. It probably won't be anywhere near a £10k premium, but it will be a premium nonetheless. Viewing it as a total cost of ownership keeps things reasonable(ish).