I have had materials from Slec in the past and I was disappointed in the plywood I received. It was supposed to be 5ply birch - but - I received 3ply. It splinters too easily when cut. Their strip material is okay; but; if I want real high quality good grade ply - I like to deal with Mantura models. They are quite expensive, but, I think if you want good quality - here you get what you pay for - fair comment the plywood size isn't that big for what you pay - but - you know you will get one of the best grades of material. Even their strip material - such as the mahogany I use for exterior planking has very few defects if any. As the old saying goes - if a job is worth doing, its worth doing right - or why spoil the ship for a hap'orth of tar.
https://www.mantuamodel.co.uk/I have also dealt with Cornwall models a few times but a few times their packing is to be desired when they post it out to you. Had one or two pieces which have snapped, not Cornwall models fault, but a size 11 boot by some mail person.
Another place that comes to mind is JoTika -
http://www.jotika-ltd.com/Pages/1024768/index.htm expensive again but good quality.
John