It’s unlikely BT lacks data limits; an unpaid bill is more probable to result in full termination. I’m not sure about the cable, but it shouldn’t be operating at just 100Mbit unless the PC uses a 100Mbit adapter. A cable problem can’t be entirely ruled out.
Speed drops can occur even without visible damage. Data limits might not be clearly shown. A sudden slowdown seems likely, but we can't confirm more details without a response from the OP.
BT does not limit data limits, period. In the UK ISPs must be much more open about their services. Could there be short-term issues or maybe a line problem since it's likely VDSL? Definitely. But it won't be an intentional speed cap except Dynamic Line Management might reduce the line speed because of a fault. A sharp drop to single digits usually occurs when the phone line gets damaged—like one wire breaking or becoming loose. I'd look for crackling sounds or no dial tone at all.