Surely that depends on whether the amount paid to the organisers above the costs to you is worth more than the time you'd spend?
I'd criticise a lot of these organised charity rides for a lack of transparency, but not simply for having a paid organiser.
Generally there is a registration fee £50 - £150 payable by you, then if you pay yourself there will be a payment option around £800
If you do it through sponsorship there is still the £150 but you have to guarantee to raise £1500 + of which the organisors will take the 'payment option value'
Two things to consider,
1) would you value a 4 day cycle holiday at £1k + (after expense of getting to and from London + some meals + expenses) if you are paying it out of your own pocket?
2) Do you think the first 160 people that sponsor you for £5 realise that their entire donation will be going to fund your 'holiday' if you take the sponsor option?
Just break the costs down, what you are getting,
£30 ferry crossing
B&B and a meal out at night, bit of lunch £50 - £100 x 4
Train from Paris to Calais £50?
Ok so you get support, if you need it then fine, but its not exactly outer mongolia is it? I found cycling in France that if I stopped at any time for any reason at the side of the road or an a track another cyclist would stop and enquire if everything was ok, regardless of nationality.
+ the OP is talking about it being their summer holiday, if in a relationship then take the partner as your support and enjoy a romantic break in Paris at the end of it for roughly the same price as the organised tour. Or take longer and cycle back. If willing to take a pannier or two you could tour for a month on that budget.