It's likely that it would be deemed that you were selling the alcohol, thus constituting a 'sale by retail' as defined by the Licensing Act 2003 and making you subject to the provisions of that Act.
A couple of (roughly) analogous situations:
1. It's illegal to sell ice cream (under local bye-laws) on Frinton beach or on the areas adjacent to it. So a trader started selling paper flags (or the type that you put into sandcastles) for the usual price of an ice cream and 'giving away' a 'free' ice cream with each flag. It was ruled that, although the ice creams were nominally free, the trader was still 'selling' them as far as the law was concerned.
2. You've probably seen the words 'no purchase necessary' associated with offers where, for example, you buy a Mars bar in the hope that there's a winning ticket for something inside the wrapper. That's because it's illegal to run a commercial lottery (other than the National Lottery) in this country and the law regards the sale of such a Mars bar as providing entry into such a lottery. (i.e. even though the chance of winning something is being given away 'free' with the Mars bar, the law still regards it as 'selling' a lottery ticket unless an alternative means of acquiring a ticket, without a purchase, is made available).
So, in both of those examples, the 'freebie' offered by the trader is regarded as a 'sale' within the law. Correspondingly your 'free' bottles of Prosecco are likely to constitute 'sales', requiring you to hold a licence for such sales or else face prosecution.