[Ignitionnet]

Yes, it's due to congestion between Brentford and Croydon. Crawley, like Mortlake where I'm connected, goes through Croydon, I just checked.

Here's a reverse traceroute from TBB and a forward one from me:

# Host Sent Recvd Best Avg Worst
1 thinkbroadband-gw2.core-rs2.thdo.ncuk.net 15 15 0ms 3ms 11ms
2 gi4-3-10-star1.bdr-rt1.thn.ncuk.net 15 15 0ms 0ms 4ms
3 gi4-47-10-star1.bdr-rt3.thdo.ncuk.net 15 15 0ms 4ms 49ms
4 linx-gw2.router.ntli.net 15 15 0ms 1ms 4ms
5 brnt-tmr-1-as0-0.network.virginmedia.net 15 15 1ms 2ms 26ms
6 brnt-bb-1b-ae5-0.network.virginmedia.net 14 15 1ms 11ms 76ms
7 brnt-bb-1a-ae0-0.network.virginmedia.net 15 15 1ms 3ms 23ms
9 mort-geam-1a-tenge71.network.virginmedia.net 15 15 15ms 21ms 60ms
10 mort-cmts-06-gigaether-151.network.virginmedia.net 15 15 17ms 19ms 21ms
11 cpc8-mort6-2-0-custx.croy.cable.virginmedia.com 14 15 23ms 32ms 41ms

traceroute to 80.249.99.123 (80.249.99.123), 30 hops max, 40 byte packets

1 cpc8-mort6-2-0-gw.croy.cable.virginmedia.com (82.43.108.1) 9.369 ms 21.926 ms 7.658 ms
2 mort-geam-1a-ge115.network.virginmedia.net (81.96.225.25) 7.463 ms 9.002 ms 8.465 ms
3 croy-core-1b-tenge81-490.network.virginmedia.net (62.30.242.57) 10.952 ms 20.364 ms 13.510 ms
4 croy-core-1a-v99.network.virginmedia.net (195.182.179.57) 8.920 ms 7.863 ms 7.730 ms
5 brnt-bb-1a-ae2-0.network.virginmedia.net (195.182.178.82) 31.984 ms 35.280 ms 32.014 ms
6 brnt-bb-1b-ae0-0.network.virginmedia.net (213.105.174.226) 39.494 ms 40.668 ms 28.906 ms
7 brnt-tmr-1-ae5-0.network.virginmedia.net (213.105.159.50) 28.072 ms 29.007 ms 32.182 ms
8 telc-ic-1-as0-0.network.virginmedia.net (62.253.185.74) 26.099 ms 26.781 ms 29.373 ms
9 linx-gw1.thdo.ncuk.net (195.66.226.240) 38.386 ms 39.034 ms 32.961 ms
10 gi0-24-10-star1.core-rs2.thdo.ncuk.net (80.249.97.9) 34.847 ms 30.883 ms 32.012 ms
11 speedtest1.thinkbroadband.com (80.249.99.123) 28.560 ms 32.360 ms 28.443 ms

Trace complete

You can't see Croydon in the first trace but you can see the jump in latency between Mortlake and Brentford, the second trace shows this to be due to congestion between Croydon and Brentford as Mortlake <> Croydon is fine.

This will affect you in the exact same way, as you'll take the same path from Croydon.

1 1 ms <1 ms <1 ms CISCOE4200 [192.168.10.1]
2 10 ms 45 ms 20 ms cpc8-mort6-2-0-gw.croy.cable.virginmedia.com [82.43.108.1]
3 9 ms 11 ms 21 ms mort-geam-1a-ge215.network.virginmedia.net [81.96.225.153]
4 23 ms 17 ms 11 ms croy-core-1a-tenge81-490.network.virginmedia.net [62.30.242.41]
5 24 ms 22 ms 11 ms craw-geam-1a-tenge71.network.virginmedia.net [62.30.242.46]
6 13 ms 12 ms 13 ms craw-cmts-01.network.virginmedia.net [82.7.205.26]

EDIT: The routing is a bit weird generally - notice I'm going Mort 1a - Croy 1b - Croy 1a to get to TBB, which makes no sense as Mort 1a is connected directly to Croy 1a and Croy 1b so could save a hop by going direct.