You're free to complete saying something unlawful because until you've said something unlawful you haven't committed a crime, but furthermore, English law on what is "unlawful" to say or do is also complicated in that many things are okay to say or do in contexts where they will not cause offence but becomes illegal if done in front of the "wrong" people or with the "wrong" intent.
As such, until there has been a complaint there often will not be a basis for saying that something was unlawful to say unless it is really far over the line.
If you were to start shouting something blatantly illegal such as chanting support for a proscribed organisation, you must certainly would not find police standing there and waiting unless they deemed it likely to be more disruptive to peace and order to stop you right away.
This expends past speech - e.g. public nudity is in the same category that isn't illegal in itself but where intent or the effect on others can make it unlawful - and this notion on relying on intent and whether or not someone present took offense rather than clearly delineating where the boundary is, is a challenge with English law because of the huge gray areas it creates.
[used "English law" as shorthand here - really it is the law of England & Wales, and much of it will be the same in Scotland, but Scotland does have a separate legal system, hence why it isn't "UK law"]