You can customize the form fields of your donation forms as you like. You only need an appropriate plugin.
For example you can utilize either the “Flexible Checkout Fields for WooCommerce” or “Checkout Field Editor (Checkout Manager) for WooCommerce” plugins to easily add a UK Gift Aid checkbox to your checkout form.
Flexible Checkout Fields for WooCommerce
“Flexible Checkout Fields for WooCommerce” supports the most important features and is easy to use. For example, I also use this plugin on this page. You can download it for free on WordPress.org. However, only text fields can be added here in the free version.
Alternative: Checkout Field Editor (Checkout Manager) for WooCommerce
“Checkout Field Editor (Checkout Manager) for WooCommerce” offers more options, but is a little less straightforward to use for my liking. The free variant also has some limitations, but should be sufficient in normal cases.