I recently bought this pair of vintage M&S trousers from a charity shop and I bought them in the way I usually buy all my chazza purchases - by eye. I never bother to look at size labels really, more by holding a garment up and sizing it up to see if I think it would fit me. I rarely, if ever, try clothes on in the charity shop - I prefer to chuck it through the wash first - so I found I have become quite adept at judging whether an item will fit or not.
A great example is this pair of trousers. Marked size 10, I loved the velvet cigarette style and cropped leg. However, had I been shopping on the high street, I would have assumed a size 10 would be a bit big, I'm a fairly standard size 8/26 waist and usually find that this size is the best fit for me. I actually thought that these looked tiny for a size 10; I thought I'd give them a try. When I got them home, I checked the care label before I washed them and realised that although they are marked a size 10, the actual waist measurement supplied on the label (remember these are vintage) is 25". They fitted me (just) but a standard high street size 10 is usually marked at a 28" waist - which just goes to show how UK sizing has changed over the years.
People get so fraught over the size in their clothing and I really think that it's quite irrelevant, nowadays, as it doesn't seem to relate to any standardised measurements and look how much it has changed over the years - by M&S's measure, a UK 8 would have had a 23" waist and a size 6 a 22" one - tiny! I am a firm believer in buying clothes that you feel fit you comfortably and that you feel amazing in; who cares what the labels say (I am only interested in the washing instructions!)
It's really interesting to see how sizing has changed and what this means for the consumer, but overall I think less emphasis should be placed on the number written on the label, and more on how the garment makes you feel.