Why do they need an election if they're already in power and their current term isn't due to expire?
Simples really, she'll have to call one at some point, the timing is tactical in that they'll almost certainly win.
I was just on Reddit, and the consensus there (and they're mostly lefties, mind you) is that the Tories will shoo it in, because Corbyn is, and I'm generalising, a generally hateful prick, useless politician, and crap.
Why? No one's really explained to us foreigners why Corbyn is so hated. We know his shadow cabinet can't stand him, but the rank and file love him and keep him in. What's he doing wrong? Genuinely curious.
Essentially the issues around Corbyn boil down to his inflexibility and his lack of polish. He spent years as a protester, campaigner and eventually somewhat out of touch back bencher.
In many ways elements of the labour membership (myself included) admire his hard and fast adherence to socialist principles and support of trade unions and they are the reason he remains in the leadership role. Unlike the tories (whose MPs select a leader amongst their number) the labour party essentially run a mini democracy amongst the paying party members, many of whom are trade unionists (me), left leaning intellectuals (sort of me after a fashion) and students (once upon a time....oh never mind). This means that the rank and file MPs can in theory be saddled with a leader whom they would not choose.
Their main concerns really are that he struggles in a number of key areas, revolving most notably his ability to garner public support outside of the core membership.
He lacks the professional appearance, does not engage even remotely with spin doctors or image consultants (often turning up to official engagements wearing second hand corduroys and sandals and sporting unbrushed hair whilst claiming parliament is not a catwalk) , struggles with his presentation skills when dealing with the press (mumbling, ignoring journalists, failing to make eye contact, refusing to play the "soundbite game") and stands firm on some fairly unpopular or even politically suicidal policies (unilateral nuclear disarmament) on ideological grounds.
He is a man of ideals and few would question that, what they doubt is that he is competent to deliver on those ideals, whether he could in fact lead a country, regardless of how earnest his beliefs may be. His lack of basic social skills or charisma plus the almost draconian approach he has to running the shadow cabinet have made him unpopular with his colleagues, with them allegedly drunkenly taunting him to his face at parties, chanting "back in the USSR" until he leaves in frustration, which obviously is divisive and fractious within any working environment.
Theresa May on the other hand is riding the populist wave on the back of being the great Brexit mastermind (despite having voted against), runs an extremely professional PR set up and knows full well her current ratings are more than sufficient to sail in.
As the Lib Dems are currently pretty much a non entity in British politics following the disastrous coalition government it's hard to see this not being a tory landslide and May knows this all too well.