Routing changes, less congestion / load balancing so it was able to take more direct route?
It could equally be cosmetic as traceroute is only ever a rough guide as you can pass over routers transparently depending on how things are configured. Plus as you can see below, hops aren't everything. Talk Talk Business backhaul seems more direct but latency is actually worse than you're getting.
traceroute to bbc.co.uk (151.101.0.81) from Zen, 18 hops max, 48 byte packets
1 losubs.subs.bng1.th-lon.zen.net.uk (62.3.80.17) 14.021 ms 21.987 ms 13.167 ms
2 ae1-177.cr2.th-lon.zen.net.uk (62.3.80.38) 13.363 ms 16.488 ms 13.444 ms
3 ip81-59.fastly-gw1.lonap.net (5.57.81.59) 26.291 ms 13.621 ms 20.924 ms
4 151.101.0.81 (151.101.0.81) 13.412 ms 26.014 ms 13.239 ms
traceroute to bbc.co.uk (151.101.0.81) from Plusnet, 18 hops max, 48 byte packets
1 250.core.plus.net (195.166.130.250) 15.372 ms 16.367 ms 15.489 ms
2 84.93.253.83 (84.93.253.83) 16.198 ms 16.361 ms 16.114 ms
3 core1-be1.colindale.ukcore.bt.net (195.99.125.132) 15.919 ms 16.519 ms 15.587 ms
4 194.72.16.60 (194.72.16.60) 16.261 ms 16.547 ms 16.838 ms
5 * * *
6 151.101.0.81 (151.101.0.81) 16.637 ms 16.627 ms 16.375 ms