This remains a sore point with me.
I downgraded some years ago, from Zen 80/20 to Zen 40/10. That made sense as whilst I initially got about 50Mbps downstream, that dropped once crosstalk kicked in, to a bit under 30/9.
That 40/20 product description was subsequently amended, to be described in terms of a 35/6 average. I continued to subscribe to that service, and still get the same speed, 27/8 or so, last time I looked.
All well and good but I feel like I am just taking it in trust that I can get 8Mbps upstream from a service sold as 6Mbps. I think Ofcom or ASA changed the rules so that ISPs are not allowed to advertise the technical specification of the product in terms of service speed capabilities such as "up to 80/20", as too many consumers were unable to understand the meaning of the words "up to". Instead, they now must simply quote the average speeds that people achieve.
Being an average, the speed that some individuals achieve will be less than average speed that is advertised, and some will achieve more. But I don't think it's easy to find out how much more you are allowed to achieve before they expect you to buy a product with a faster average.
Bottom line though, in your case, chances are you'll be fine with the downgrade and see no difference. I've been fine with Zen's downgraded service, even though I exceed the average.