Can I reactivate a phone number less than 8 months after deactivation?

Unfortunately ​@gwalz unless you have transferred your number to another network (via requesting a PAC code) or reactivated your current account within 6 months then it seems that ‘your’ number would be recycled.

This is what an internet search provides for your situation:-
‘After 6 months of inactivity, most mobile phone networks will deactivate your SIM card and the associated phone number, potentially leading to the number being recycled. This means you will lose your number and any remaining credit or balance on your account. While some providers may offer a grace period to reactivate your SIM, it's generally not possible to recover the number once it's been disconnected and recycled.’

If you wish to see if this is the case then contact iD Mobile via the Live Chat:-
https://www.idmobile.co.uk/live-chat
Type ‘talk to a person’ in the dialogue box to bypass the inane questions.
Just be aware that the agents are dealing with many chats at a time and it may take a while to start a chat and also to reply to you so set some time aside to cover this.

If your number is still available in the ‘recycle’ list then you may be able to get it back BUT it (more than likely) may be the case that you won’t and will have to get a new one.

Good luck x

Hi ​@gwalz  I am sorry but its highly likely the number has already been recycled.

You can try as ​@Decembersangel72  has suggested though