Beware of forming opinion on the law from internet guides rather than the actual legislation. In the UK the legislation is the Road Traffic Act 1998 with amendments contained within the Railways and Transport Safety 2003. The legislation has two sections which cover the requirement to submit to testing.
The first is the requirement to co-operate with any one or more preliminary tests administered to the person by a constable and a person commits an offence if without reasonable excuse he fails to co-operate with a preliminary test.
The second concerns specimens where a constable may require the person to provide two specimens of breath for analysis by means of a device of a type approved by the Secretary of State, or to provide a specimen of blood or urine for a laboratory test. There is a choice but it is the constable's choice although there are other sections which place limitations on that choice. A person who, without reasonable excuse, fails to provide a specimen when required to do so is guilty of an offence as long as the constable warns them that a failure to provide it may render them liable to prosecution when they make the request.