Below are a number of people that are worth emailing on this issue, should you so wish. I have listed them here in what i think is priority order in terms of actually having and will do something about this. Please feel free to mention this website in your correspondence.
Michael Gove MP
The local MP for the area is Michael Gove MP. If you live outside of the area but are unhappy about the closure then write to your MP (https://members.parliament.uk/members/Commons).
(You will need to prove you live within their constituency so please include your full name and address in your email to your MP).
I have placed a template email here in case you simply want to copy and paste. However, I urge you to make your correspondance personal about how the closure affects you and those around you. As an MP, they are duty bound to respond to you.
Michael Gove also has a website https://www.michaelgove.com/
Ben Wallace MP (Secretary of State for Defence)
Ben Wallace MP is the Secretary of State for Defence. You can email him, in the capacity of defence secretary, regarding this issue. Whilst emails to him will be passed on to the MoD, it is not unhelpful to fill his parliamentary inbox with letters apposing the DIO’s actions. Please make it clear in your email that the matter is for him as Secretary of State for Defence so that it isn’t excluded as not from one of his constituents.
Senior DIO Officer (Lt Col “Dickie” Bishop)
Lt Col Dickie Bishop is the boss of the DIO for the South East of England based out of the Longmoor camp. The DIO’s moves to remove public access to large swathes of land, used by the military, is under his command. So you could write to him and let him know of your frustration and even (if you are so minded) to let him know that you wont stand by and let the DIO succeed in their plan. We know this issue and has been escaleted above him to central command, but we dont have email addresses of anyone higher than him.
Ash Parish Council
Whilst APC have a duty to ensure that the byelaws are followed and they do represent local residents, my experience of them has been one of little action. The will speak with the DIO, believe the DIO, allow DIO posts to be put on their website but dont seem able to hold the DIO to account. Please do email them but in my experience they will be slow to respond, if at all and are unlikely to take meaningful action. Nigel Manning is the Chair of Ash Parish Council.
Email: Nigel Manning firstname.lastname@example.org
You can also find contact details of other councillors here https://www.ashpcsurrey.gov.uk/person-category/parish-councillors/