There is no doubt that the company would use it for marketing purposes afterwards. I am amazed that companies can get round the Data Protection Act in this way.
Yes, you can no doubt unsubscribe but it may be too late - the email address may have already been shared.
I also never complete marketing surveys. A few years ago I started completing one with an offer for those who completed it to be entered into a draw with a very attractive holiday prize.
The questions were highly intrusive - what is the household income, how many children do you have, how many cars in the household, where do you like to go on holiday, etc.
It rapidly became obvious that the information would be perfect for marketing purposes. The double negatives and wording of the opt-out for them sharing the information was very confusing and I wasn't sure whether to tick the box or not.
The message is the same as that for all scams: be very wary of giving out personal information that identifies you and never provide your contact details.