Mirror dinghy kits from the 1960s and 1970s were made from 3/16" gaboon marine plywood (well, the outer two layers were gaboon). As plywood production went metric this thickness became harder and more expensive to obtain. 3/16" equals 4.6mm approximately, so nearest metric size, 5mm was adopted. Sorry, I don't have a date for the switch in the UK, but by the mid 1990's kits were 5mm gaboon marine ply.
Gaboon (sometimes called Okoumé) is widely regarded as producing the lightest, strongest plywood racing hulls.
At the time of writing (April 2014) there are class rules (part A rule 7.5 and a list of panels right at the end of the rules) regarding replacing hull panels (i.e. they have to be purchased from a Licenced Wooden Kit Manufacturer). The Class is trying to get these rules relaxed, but until that happens, if you are repairing a boat which will be raced, you need to comply with this rule. If not, or it's not a "hull panel" you are replacing, or you are patching a hole, you can use locally sourced wood.
5mm ply is not a "standard" plywood thickness, in contrast 4mm and 6mm which are and thus commonly available. Robbins Timber in Bristol sell 5mm gaboon in their Robbins Elite range. There are other possible manufacturers of 5mm Gaboon for example Joulbert Plywood (France), Bruynzeel Multipanel (Dutch) & Corà Domenico & Figli - S.p.a. (Italian). It's just going to depend on what is locally available from your timber merchant, but please make sure it is marine ply to BS 1088. If you can't get 5mm, you can always plane down some 6mm ply, or may be able to make do with 4mm for some of the lightly stressed parts (like the bow shapes).