Well I'm not a British driver, and my Isettas have 4 wheels, but I have acquainted myself enough with the driving dynamics of 2 in front and 1 out back three wheelers well enough to report the following: You want to choose a tire combination that will allow the front end to wash out before the rear wheel sliding, out around a corner to minimize lateral weight transfer to the single wheeled axle, as that is the axle that possess the least ability to resist roll.
Therefore tire height and width should be reduced on the front end to produce a smaller contact patch that will more readily lose traction than the single rear wheel if that is possible.
To some degree, the same effect can be achieved by raising rear tire pressure- sometimes as high as double the front pressure- let your backside seat of the pants be your guide as to how much higher to make the rear tire pressure. The higher rear pressure and the resultant stiffer sidewall in the rear tire also creates the additional benefit of increasing the roll Resistance at the rear axle. You can demo this for yourself by standing alongside your single rear wheel and pushing sideways near the top of your car while watching it's sidewall. You will undoubtedly notice that as you rock your car sideways, the sidewall on the side that weight is being transferred to bulges outward momentarily in reaction to this. You will also notice that this effect increases significantly at lower pressures, and decreases proportionately at higher pressures.
Considering all the above, you will want to install the widest stickiest tire with the lowest and stiffest sidewall at the rear of the vehicle, and pump it up to a significantly higher pressure to boot. Radial tires usually have significantly stiffer sidewalls, and therefore should be preferred at this location.
You also want the skinniest front tires running lower pressures to encourage wash out prior to the rear ends performance of same. Bias ply tires are typically more likely to exhibit this tendency. So it seems that I have put the shoe on the other foot, so to speak, in that I have reversed your friend's recommendation.
My self modified HMV Freeway and prototype Badsey Bullet both successfully manifest the ideologies to an extreme in that they are equipped with a tire size differential of plus 3 and plus 5" respectively when comparing the tire sizes at the rear to the fronts.
Perhaps your friend's recommendation was more influenced by availability of conventionally sized tires, and reasons of ride character, rather than overall cornering, handling and safety in avoiding roll over/ loss of controll.
