Going from 195/50/15 to 195/55/15 is a really minor change and, as others have said, you'll be absolutely fine.
Quick breakdown for anyone unsure on the numbers:
195 = tyre width (mm)
55 = sidewall/profile height (as a % of width)
15 = rim diameter (inches)
You're only stepping up 5 points in profile, so the overall rolling diameter increases slightly nothing that'll cause rubbing or clearance issues if the car's on standard suspension. On stock springs, 195/55/15s will sit and drive just fine, no MOT or legal concerns either.
The only thing to watch is if the car's been lowered in that case, you might get a touch of rubbing on full lock or over bumps, and rolling the arch lip slightly sorts that out. But since you've said everything's standard, you shouldn't need to touch anything just get them fitted and enjoy the slightly plumper sidewall (a bit more cushioning over rough B-roads and speed bumps, which plenty of MK2 Cabriolet owners actually prefer).
Nice find on a set with 7mm tread left too good result.