My mum has a leasehold flat which has about 86 or 87 years left on it. We want to extend the lease before it gets below 80 years and have been offered two methods by the freeholder; formal and informal.

Formal:
Employ your own surveyor and serve notice via a solicitor with the surveyor's quote.

Informal:
Fill out a form, get a price, pay their legal fees.

Is there generally a price difference between these two routes? Or is there a meaningful benefit by choosing one route Vs another?

If it makes a difference we would probably opt for a 90 year extension rather than topping it back up to 125 years.

Thank you in advance for the help and apologies for butchering the summary as I'm sure it's worded different for those in the business.