I have ms 365, is there a licence to cover just excel and word and ms mail as these are the only the only parts I use ?

For home use, there is no Microsoft 365 subscription that licenses only Word, Excel, and Outlook/“MS mail” separately at a lower price. The home subscriptions are bundled:

• Microsoft 365 Basic: does not include desktop apps like Word or Excel; it only provides web and mobile versions plus OneDrive storage. It cannot be used to install the full apps on a PC.
• Microsoft 365 Personal / Family: include the full desktop versions of Word, Excel, PowerPoint, and Outlook, plus web and mobile apps and cloud storage.

Because of this, to have the full installed versions of Word, Excel, and Outlook on Windows, a Microsoft 365 Personal or Family subscription is required; there is no cheaper “apps-only” home plan that excludes the other components.

If only Word and Excel are needed and a subscription is not desired, a one-time purchase of Office (for example, Office Home 2024 or similar perpetual Office license) can be used instead of Microsoft 365. That gives perpetual rights to Word, Excel, and other included apps on a single device, but without the subscription benefits.

The btinternet.com email address can continue to be used in Outlook regardless of the plan, as long as Outlook is licensed (via Microsoft 365 Personal/Family or a perpetual Office license) or accessed via the free Outlook app where applicable.

References:

• What's the difference between a Microsoft 365 subscription and free web apps
• No license associated with my email account - Microsoft Q&A
• Word program without annual subscription - Microsoft Q&A