Then many people are wrong, unless they can demonstrate that the judicial outcome is unjust, unfair or improper. If the Judge has followed the law, and sentenced according to the guidelines, there is no miscarriage. In the UK a miscarriage of justice is usually considered to be where an innocent person has been wrongly convicted.Many people consider an overly lenient sentence to be a miscarriage of justice.
You might also consider that the sentence is unduly lenient. Sentences can be appealed but again, it the Attorney General's office has to be satisfied that the Judge has erred in law / failed to correctly follow the sentencing guidelines. This is rare. The last few Conservative AG's used their post for ill informed grandstanding:
The ULS regime applies only to sentences that are unduly lenient and not to sentences that are simply lenient. A sentence is unduly lenient:
“… where it falls outside the range of sentences which the judge, applying his mind to all the relevant factors, could reasonably consider appropriate” (Attorney General's Reference No 4 of 1989 [1990] 1 WLR 41, Lord Lane CJ).
In Attorney General’s Reference Nos 3 and 5 of 1989 (1990) 90 Cr App R 358 the court agreed with submissions that to allow a reference there must have been some error of principle in the judge's sentence; that, in the absence of the sentence being altered by the court, public confidence would be damaged; and that the court should only grant leave in exceptional circumstances, and not in borderline cases.
In Edwards [2012] EWCA Crim 2746 the Court held that the scheme is designed to deal with cases where judges have fallen into gross error.