The usual and safest legal frequency for model aircraft is the 35MHZ band. Modern RC aeroplanes are not toys, you cannot fly them on parks or recreation grounds unless they are specifically controlled sites. The British Model Flying Association website will give you details of a recognised club near you, and can provide third part insurance cover.
27MHZ can be used for planes, boats, cars and CB radio but is risky.
40HHZ is for Boats and Cars ONLY, and is dangerous and illegal if used in aeroplanes.
49MHZ is a 'toy' frequency for SHORT RANGE ONLY