In UK, as I understand it, unauthorised access to computers and networks/communications under the Computer Misuse Act is a strict liability situation: you only have to do it to break the law, no harm needs to be shown. MitM-ing my connection would be unauthorised access. It's a crime.

Should there be some form in intention, I wonder?

There is also negligence. Like in the original case [1] where a woman got ill and suffered emotional distress after drinking bottled ginger beer with a decomposing snail in it. Even though the manufacturer had no intent to sell "snail beer" -- a consumer has a trust relationship with them: in exchange for money they can reasonably expect the manufacturer to take enough care not to bug them.

[1] http://en.wikipedia.org/wiki/Donoghue_v_Stevenson "it was reasonably foreseeable that failure to ensure the product's safety would lead to harm of consumers."

According to the CMA [0];

The offence of unauthorised access requires proof of two mens rea elements, (see section 4 CMA):

    (1) there must be knowledge that the intended access was unauthorised; and
    (2) there must have been an intention to obtain information about a program or data held in a computer - section 1(2) CMA.

[0] http://www.cps.gov.uk/legal/a_to_c/computer_misuse_act_1990/

See also http://www.legislation.gov.uk/ukpga/1990/18 - the current unauthorised intercept of data, installation of a MitM system appears to be an offence under inter alia Section 1, 2 and 3 separately.