Assuming you buy the services and products beforehand and then provide them to your clients.
Should the cost of these products and services be deducted from my declared income or do I include them and then claim them as allowable expenses?
You arrive at your final income after accounting for your incomings and outgoings ?
regularly buys products and services on behalf of clients
These are your expenses.
invoice them for these costs after
These are your earnings.
These are not exactly allowable expenses, but more as the cost of doing your business, so it will be deducted from your earnings. There will be other business expenses which you need to deduct from your earnings and then you arrive at your income/profit. So before you arrive at your income all allowable expenses have been deducted.
include on my invoices to clients
VAT if you charge VAT. Any charges you require them to pay i.e. credit card charges etc. You don't need to inform clients about any costs you incur for doing your business unless required by law.
If you are unsure about something browse the gov.uk website or obtain the services of an accountant. Accounting issues might be costly on your pocket if mistakes are committed.