You have no add latency like added by interleaving.
You have retransmission (aka G.INP) which adds no latency and any corrupt data is retransmitted by the DSLAM (for downstream, your modem does the upstream).
See here:
https://kitz.co.uk/adsl/retransmission.htmDepending on your location in the country and the ISP routing involved your "ping" to uk sites should be in the region of 10-20ms.
Those nearer London or on brilliant ISP routing can have 5ms pings.
In Central Scotland the lowest ping I've seen on a domestic FTTC line is 15.1ms.
Scottish highlands can be 20ms+.
The further North you go the higher the ping (generally) as most ISP's go through London.
Low interleaving adds 8ms on FTTC.
What is your lowest ping to the wider internet?