If you know my postcode, then you also know my postcode location (lat, lng).
If I now want to search for Garden Centres (or whatever) whose location (lat, lng) falls inside a circle then you are going to want an sql search like this one.
If this is just a bell or a whistle on the website you are developing, then you should be fine.
If it turns out that this is generating a lot of traffic (and server-side computations) then seriously consider using Mysql's native spatial abilities by not simply storing lat, lng pairs but POINT(x,y) as well. You should get selects a lot quicker using mysql spatial point -- but as I say, don't bother unless you see proof that you need to learn this -- unless it turns you on of course ...