Where in the country are you?

We are in the Birmingham/Solihull area

Ok, Im not familiar with this area, so this is what you need to look for.
The plywood substrate of an engineered floor should be birch, particularly if you are fitting over underfloor heating, although some will recommend you don't.
Look at the plywood part of your samples. It should be uniform and flat, this is as important as the topping of oak. Any buckles, rule it out. The oak layer is generally 3-8mm thick.
Go for as thick as you can afford. Future sanding and refinishing no good on 3mm.
You say you want matt varnish, I would consider Matt hard wax oil. This will give you a durable Matt finish however it is much easier to sand and refinish small areas of damage in the future.
If you need any more help message me

Haha.. We went thru something very similar to u! Ordered 30 different samples, 10 different shops which we visited many times. In the end we went for a long and wide plank of engineered oak which is smoke and uv oiled. Personally I think u should rule out anything that looks too narrow, too short and doesn't have a good 3mm+ wear layer of oak. Our we're particularly thick -at 18mm ply and 4mm wear layer..with planks 2m long and 20cm wide. Will look good when layed

Beautiful flooring ^

Hi - the advice given by Judith Harrop is sound. The first question we ask is, "what are you laying the floor on to?". The reason is that an oak floor can be laid 3 ways; glued, floating or nailed. The type of floor may be different depending on your subfloor.

For example, if you were laying on joists, the floorboard needs to be structural and therefore 20mm. If going on UFH screed, a 15mm engineered board glued down would be good.

Another consideration would be the flooring finish. Is is unfinished, oiled, lacquered or varnished. Be sure to know exactly what product is on the floorboard as future maintenance and repairs may be crucial in knowing this.

See here