Which third parties?
It would help a lot if the government gave some clearer indication of which 3rd parties would be allowed to see the data.
I volunteer to work with a UK charity and we do get to see some data from GP surgeries - collected with full agreement not stealth slurped - and the data controller is HSCIC.
In my experience HSCIC has been a tough data controller very aware that data has to be anonymised before passing out of its direct control.
So if, for instance, the aim was to provide a cancer research charity with data on how effectively recommendations were being followed at the GP level for the treatment, support and referral of newly diagnosed and long term cancer patients then there would probably be general support for a data extraction exercise.
This could be used, for example, to identify 'postcode lotteries' and build a coherent campaign.
Pick your own chronic disease and charity - you can usually see major benefits for patients in the medium to long term when interested campaigning bodies get carefully anonymised data from GPs, A&E, specialist clinics etc.
Pick your own bad example - much quoted is the Insurance Company which wants data which can be reverse engineered to identify individuals and then asses them for long term risk - and you can easily see that there is no way that this data should ever be released.
So an open and honest government would publish a list of all recipients of the data alongside a charter to HSCIC which ensures that the data is always anonymised before being released to outside parties.
The list would be rigorously maintained and audited.
This would go at least some way towards building confidence in the safety of releasing your personal data.
Of course, you would also have to trust current and future governments not to change the rules.
Best to have the data held by a trusted third party not directly under government control, but not under commercial control either - possibly a charity.
However, once the data is out there and aggregated it cannot be recalled which does require an enormous amount of trust from the UK population.