It's difficult. So many people run races for charity now that, at this time of year as people are just starting to get into the longer runs of marathon training, you can feel besieged with being asked for donations.
I can't afford to sponsor all my friends and colleagues so tend to only do those ones who have a particular personal story which is driving them to do it. If it's just someone who wanted to do the marathon but couldn't get in through the ballot then not so much, I'm afraid.
The problem you are also facing is that the minimum sponsorship for London is really high (usually). I know it varies from charity to charity but lots of them are around £2k which - unless you have a huge amount of support behind you, a real story to tell or very rich friends - is going to be very hard to achieve.
Shelter, for example, asks for a £2,250 fundraising target for the London Marathon but £500 for Manchester ... so when friends have been fretting about hitting London targets in the past I do wonder why they didn't just choose another marathon!
If I were you I would stop sending WhatsApp links to friends (awkward or what) and be a bit more creative.
First up, check if your work does match funding. Lots of private sector companies do, so that's an easy way to double your sponsorship straight away.
Then think about things you can do which aren't just straight-up asking for money. If you bake, do a bake sale at work, for example. But as a pp said, the charity you're running for will usually have sent you some good ideas. You need to move this away from looking like you're being paid to do something you wanted to do anyway. If you have a personal story to tell that your friends and family will empathise with - tell it. If it's truly exceptional then tell the local press too, sending them photos of you training to go with the story.
And if you have committed to a very large fundraising target because you wanted to do London and not another race, then you probably do have to suck up the commitment of topping it up yourself.