When probate is needed
It is usually needed when the person owned property in their sole name, or held significant sums with a single institution. Each bank sets its own threshold, so it is worth asking them directly before assuming.
Doing it yourself
Many people apply online through GOV.UK. There is an application fee, and additional copies of the grant cost a small amount each.
A solicitor typically charges either a fixed fee or a percentage of the estate. Percentage fees on a larger estate can be substantial, so always ask for the basis of charging in writing.
Inheritance tax
Most estates pay no inheritance tax, but you may still need to report the estate's value. The rules change, so check the current thresholds on GOV.UK rather than relying on older advice.