Not always, PP, I've been struggling with a package this morning. If it were 1-2kg it would be cheapest with Royal Mail (too big for Collect plus and costs 80p more at that weight with Myhermes).
Luckily it came in at 800g so Myhermes cheapest after all, but only because I want tracking/signature. If it were worth less and tracking no so important I would go for the lowest Royal Mail price band as best buy.
It's been a pain.
I'm also sending a huge 21 kg parcel tomorrow and Parcel Force will be cheapest by a big margin for that.