Congratulations.
Cabinet Office is likely to want you to hit the ground running.
Be proactive, get onto the intranet and find the basics e.g. HR/payroll system, room booking, orientation maps of the buildings, IT systems, fire evacuation process etc. Staff Handbook, HR & Facilities policies, cost centres and acct codes etc. Do your mandatory training ASAP e.g DSE.
A lot will be familiar but there will be differences, don’t be afraid to ask is this the policy/process or do you do something different. Don’t be surprised if it’s quiet the first day/week or two it will ramp up very quickly.
Before you start familiarise yourself with the gov.uk Cabinet Office website, you can find the business plans etc. to give you a good idea what their priorities are.
If you can try to go in for a day/half day to meet the team, see where you are sitting and to get system log ons, pass etc. It really helps if you have at least an email and Intranet access on day one. If you can get your IT kit before you start.
Sorry that’s a bit of a brain dump, but gives you some pointers.