There is guidance on the government website about time spent working for a charity at this link.
And at this link they say:
If your company temporarily transfers an employee (secondment) or an employee volunteers for a charity in work time, you can deduct any costs as normal business expenses.
Continue to pay the employee (which can be yourself) and run PAYE on the salary as usual. You can deduct the costs (including wages and business expenses) from your taxable profits as if they are working for you as normal.
Record the hours spend in secondment or volunteering as you would normally within your timesheets to enable you to pay them as normal. Ensure the time is allocated to the Charity rather than your clients.
Note that the Government website says that You can’t claim the costs of employees on secondment or volunteering at a community amateur sports club (CASC) and you can check if your local charity team is one if it is listed here.
About monthly hosting costs, the first link also says
Your limited company can pay less Corporation Tax when it gives money to a charity or community amateur sports club (CASC).
Deduct the value of the donations from your total business profits before you pay tax.
If the Ltd company pays the hosting fees for the charity’s website, I’m not sure how that is different from the Ltd company donating the same amount of money to the charity for them to then pay the hosting fees?
Hope this helps a bit.