From the perspective of the ISP it's a no brainer to offer fixed rate services as they carry less potential cost liability for the ISP on a line that may sustain higher speed on MAX.
Technically most ISPs can provide fixed rate (512/1M/2M) services BUT there can be issues with availability on some exchanges AND we are seeing reports from BTw that it won't be supported on 21cn (although we've pointed out we'd like to keep the service as its suitable for many of our business customers).
Those wanting Enta based services can get non-MAX service on the services usually supplied over MAX.
Those registering with UKFSN can select the Legacy ADSL option to request this. That way you get the reliability of a fixed rate (non MAX) line with the pricing advantages of the Allowance or PAYG service.