It is 'should'.
For context, these are other'shoulds':
You should wear
a cycle helmet which conforms to current regulations, is the correct size and securely fastened
appropriate clothes for cycling. Avoid clothes which may get tangled in the chain, or in a wheel or may obscure your lights
light-coloured or fluorescent clothing which helps other road users to see you in daylight and poor light
reflective clothing and/or accessories (belt, arm or ankle bands) in the dark.
Not riding on pavement is a 'must'
http://www.highwaycodeuk.co.uk/rules-for-cyclists.html