I don't think the Electronic Communications Act 2000 (https://www.legislation.gov.uk/ukpga/2000/7/contents) is relevant here, although perhaps you are referring to some other legislation?
If the forms constitute "business letters" under the Companies Act 2006, then Chapter 41 of that Act requires the disclosure of an address in the UK at which service of any document relating in any way to the business will be effective (https://www.legislation.gov.uk/ukpga/2006/46/part/41).
If the forms are part of an "information society service" and you are concluding a contract by means of the web forms, then you would have an obligation to supply geographic address under the Ecommerce Regulations (http://www.legislation.gov.uk/uksi/2002/2013/contents/made).
As you have noted, the GDPR also requires that you publish contact details.
There are now many separate pieces of legislation requiring that business names and addresses be published in various places; there will very likely be more such legislation in future. Accordingly, it usually makes sense to publish this information wherever practicable, even if there is no current legal requirement.