MarkE
As one who works as a FI and loves it - a word of caution. As has been said, doing the minimum will limit you to basic flight instruction which is great if you can live on £500-800 per month after tax. Living wages can be made if you progress to CPL/ME/IR instruction but you can't do this if you don't hold the relevant rating yourself.
The poor pay for basic FIs is due to the fact that for many newly-qualified CPLs it has been a useful way to keep flying and building hours while looking for their first commercial position. More senior instructors are paid properly because they are harder to replace and actively sought, particularly by light commercial operators.
We do, however, live in interesting times for the industry, as many 200 hour frozen ATPL holders now choose to fund their own MCC and Type Ratings (£25k ish) and go straight to the airlines rather than spending £5-8k on a FI rating and building some GA experience first. The pending Multi Crew licence route will further reduce the advantages of flight instruction as a means to an end for a budding airline pilot and in my opinion this will mean that schools will ultimately have to employ and adequately reward full-time career FIs if they want to stay in business. Don't hold your breath, though!