1. Marks and Spencer shirts are what I swear by for work, but also for wearing cas. They’re not big or flashy, but are good value for £25, giving you a slim-fit Oxford shirt with a button-down collar. Comes in a range of decent colours (well, white and blue, as well as some stripes) and goes well with pretty much anything.
2. Been put onto this by my girlfriend - some relatively strong polo shirts available at New Look for a very decent price (about £13). I like the bright colours, but also the three button situation and the high collar on these particular ones. Having bought this grey one already, I can attest that the quality is relatively good.