This Freedom of Information request highlights the DVLA's view on why they can hand out your personal details to anyone that asks. However if you fear you will be harassed, you can request the DVLA does not hand them out. You may want to write to the DVLA if you are concerned. They say:
Regulation 27 (1)(e) of the Road Vehicles (Registration and Licensing)Regulations 2002 permits the disclosure of vehicle keeper information to those who can demonstrate a reasonable cause for needing it. Reasonable cause is not defined in legislation but the government’s policy is that it should relate to the vehicle or its use, following incidents where there may be liability on the part of the driver. Guidance on what constitutes reasonable cause is published online at http://www.gov.uk/request-information-from-dvla
The law allows DVLA to disclose keeper details, from the vehicle database, to law enforcement authorities or private litigants as a first point of contact to establish where liability for an incident or event may lay. Only the registered keeper can then decide whether he/she is able or willing to assist. Disclosure in these circumstances does not breach the Data Protection Act (DPA) and the Information Commissioner’s Office is fully aware that the data held on the DVLA vehicle database is released in this way.
The DVLA takes the protection and security of its data very seriously and has procedures in place to ensure data is disclosed only where it is lawful and fair to do so and where the provisions of the Data Protection Act (DPA) are met. The Agency provides vehicle keeper details as a first point of contact to allow the driver to be identified.
The DVLA would not be able to justify withholding personal information from third parties able to demonstrate reasonable cause without evidence that such disclosure would cause unwarranted and substantial distress or damage.
We'd recommend that you collect some of the available information on the harassment methods these companies use and write to the DVLA and your MP/AM/MSP about your concerns.