I'm not a Zen customer, so may be wrong on this, but I very much doubt that Zen lock the Fritzbox down.
You are paying for more than basic connection with Zen, you ar paying for a better level of support and investment in their own infrastructure.
You only have to look at the ISP Review ratings on the main TBB site to see how differently people rate Zen compared to TT.
By the way the sync speed you are seeing is nothing to do with the ISP, it's down to the physical properties oy your line minus any crosstalk fromother people's FTTC connection in your cabinet. Throughput is a different matter and the consumer ISPs like TT tend to run their networks at higher capacity levels than the likes of Zen.
You tend to get what you pay for in life and we all have different priorities and budgets.
Doing a cost comparison isn't comparing like for like in my view. I'm not a Zen customer and definitely not a Zen fan boy, but I do think your comments need to be put into context.