The beauty of a Monocular is their small dimensions and portability.
Their downside, is their small magnification compared to a good pair of binoculars, and unless you buy one of the top models, such as the Leica Monovid or similar spec model from Zeiss, then the optical quality is relatively poor.
A monocular will never outperform a decent set of binoculars, as their magnification is too small, but if its just a small very portable eyepiece your after, then a Monocular makes a useful addition to a pair of binoculars.
Their very useful for bird watching, where you want instant recognition of a bird, at fairly close proximity, and there is insufficient time to reach for your binoculars, focussing them, and adjusting them for comfort.
The best all rounder would be a pair of portable 8x 20 binoculars, and there are so many on the market, that your almost spoilt for choice, and £50 would get you something pretty decent as well.
Simple matter, is to buy the very best you can afford, and there are some decent ones out there for around £50, but your better off investing in a small pair 8x20 binoculars, which will be very portable, and lightweight as well.
A monocular is more of a specialist instrument, as there are very few models on the market, compared to literally hundreds of binoculars of all sizes, weight and magnification.