The only time (other than due to capacity issues) BT will insist on ADSL over fibre is when there isn't an up to date survey for your property - such as in the case of a brand new building.
I had to place an order for a phone line and ADSL with my ISP and wait for it to be activated before the survey (done during the process) was updated on the Open Reach database and fibre suddenly became available. My ISP was fine about upgrading my internet from ADSL to fibre just a week into my 12 month ADSL contract with no charges.
ADSL is always offered because its pretty much ubiquitous these days - you have to be fairly way out in the sticks for it not to apply, so if you are in a postcode region which offers ADSL then ISPs can offer it without an up to date survey. Fibre however requires an up to date survey - insist your ISP request one to be done if you think you should have access to fibre.