a11y keyboard navigation and screen reader improvements

Some accessibility fixes I'm planning on patching when I've got some time, jotting it down as a start...

• Skip to content for keyboard navigation (something like GDS) WONT FIX : this is avaliable in v4
• Missing aria-label
  • Search box input
  • Breadcrumb (follow NHS / APG) -- also change div to nav
• SVGs add role="presentation" when pre-described by other content
  • Top nav grouped dropdown, chevron icon
  • Light/dark mode icon
  • Sidebar collapse icon (more investigation needed to see if this is actually required)
• ARIA hidden element must not be focusable or contain focusable elements
  • "Scroll to top"