The answer is surprisingly little.
The problem was that 2019 British parliamentary election was a referendum on Labour Party Leader Jeremy Corbyn. And Britain spoke loudly, and clearly.
Corbyn shouldn't have agreed to the election in the first place. Prime Minister Boris Johnson was losing every vote in parliament. If Labour, LibDems, and the disloyal Remain Tories had kept that up, then they could driven BoJo’s poll numbers to the ground and have turned this election into a referendum on BoJo.
The Liberal Democrats under the guidance of the inexperienced Jo Swinson were firmly Remain, yet she proved during the campaign that she was not ready for Leadership. If Labour had taken a clear position against Brexit then this election would have definitely been a referendum on Brexit.
I've run the numbers on Britain. If Labour, LibDems, SNP, Greens, Plaid Cymru, and Social Democratic & Labour Party had run as a “Remain" coalition against a Tory & Brexit Leave coalition, then Remain would've won 2 million more votes than than the Leavers. Yet Leave would have still won a 327-308 parliamentary majority. Sound familiar?
Now you cannot equate the British Leaders with American politicians. According to YouGov, Corbyn's favorables were 40 points underwater with universal name ID. By comparison BoJo was just 12 points underwater with universal name ID. And while Swinson was just 2 points underwater, well over half the country had never heard of her and therefore were not going to vote for Liberal Democrats.
By comparison our candidates are far more favorable:
Source: YouGov