You could check and, if they are worn down, change the brushes - easier than swapping out the alternator at the roadside.
As a get you home job any replacements will do as long as they fit - I've filed some larger non related ones down in the past and not soldered them in just fitted them in front of the worn out brushes - worked a treat.
Ilford to Brighton even with a fully charged battery in A1 condition may be pushing it though without the alternator. If you had a second fully charged battery to hand and kept all the non essentials switched off then worth a try.
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