If you are "worth" £10 as an employee, then you could probably assume you actually cost the employer more. A rough calculation:
£10 x 35 hrs x 52 weeks = £18,200.
Employer NI = £125 (is dependant on lots of things but I did a rough calc and lets call it £125 per month)
Pension @ 4% = £910 per yr
So £18,200 + £1500 (£125 x 12) + £910 = £20,610. That is £11.32 an hour. But you should also include the costs of sickness, holiday (ie on productivity), other benefits and training budget. So at least £12 ph.
Does the prospective client know your previous pay? They could well assume you were being paid more etc.
I think you also need to think about whether or not this is the start of a VA business. If it is, you need to think in terms of saving 25% for tax/NI (assuming you earn enough) expenses etc which will all eat into the hourly rate.
I personally (and it is very easy for me, a stranger online!) think you should go in at £15 per hour and see if they knock you down? Finding a reliable consistent person is very valuable. I would imagine most businesses would expect to pay that sort of rate. I don't think it is unreasonable.
There have been lots of threads on here where a lot of us (me included) all started off too low. Hate to say it but a man might well say "I am worth much more than I was paid previously, I'll ask for £25"!
Good luck.