1. For the best part of a decade, Western politicians forced a fixation on Iraq and Afghanistan that was so totally all-encompassing for the security services and armed forces that attention to, and expertise on Russia was allowed to dwindle to almost zero** (and not just in the UK). The scrabbling around to rectify this began a few years ago, but there was an immense, gaping window of opportunity for Russians to exploit - and they did.
2. Of course, it's handy that Corbyn was a blatant, card-carrying Useful Idiot, operating in plain sight on RT and on his feet in the Commons. (Suggesting that the Russians be given our Salisbury Novichok evidence for their review and considered response ... - for pity's sake. It might even - justifiably - have cost him the 2019 GE: not many voters have any truck with that kind of crap.)
But. There are some truly masssive scandals for the Conservative Party lurking out there. It'll be curtains for some. (Should be, anyway.)
** Belated update for the record, from the Graun:
... the agencies’ post-cold war workload ... was dominated by counter-terrorism, especially in the wake of 9/11, as well as ongoing issues related to Northern Ireland, and the mounting threat from far-right violent extremism. By 2008-09, according to the Russia report, just 3% of MI5’s effort was allocated to work against “hostile state activity”. By 2006, just 4% of GCHQ’s work was focused on the former Soviet bloc, for similar reasons. These proportions have risen again recently, although the precise figures have been redacted from the report. Nevertheless, they mean that Putin’s Russia was largely faced with an open goal.