FTTC requires the cabinet to be enabled, and this isn't done for all cabinets within an exchange area, so you do need to take care.
Use the link given by someone else, it is definitely better than the Infinity one for the reasons given.
The only problems you will then encounter are:
a) The speed estimates come in a range that covers the entire postcode area.
b) When a particular postcode is served by 2 different cabinets (or, worse, 2 cabinets on 2 different exchanges). You might not get service at some of the properties.
There may be other special cases where a particular line cannot support FTTC, depending on the equipment already installed for voice (eg ISDN lines, or TPON). You may not see this distinction on a postcode search.
The "phone number" search is definitive, so you should request help from the letting/estate agent for a final confirmation before signing anything.