Speeding fine for exceeding tempoary 50mph on a motorway

Hi guys I got a notice through today for Exceeding a temporary speeding limit on M4 J19 eastbound.  I was driving a hire car at the time. What annoys me is I normally make sure well with in the speed limit  when I've seen those before.  I was doing 59 when the limit was 50.  To be honest I assume it was on gantry above the motorway.  As there where no road works etc.    I was driving back from Wales at the time. It's not a road I use very often thought that no excuse. 

I've been advised this is 6 points or disqualification at the discretion of the courts and a fine.  While I expected points I would have expected only 3.  Which is what the RAC website says for speed was going in comparison to speed on road at the time. 

Don't suppose there much point asking why it's 6 points and not 3 points is there.
Is disqualification likely and it's saying up £2500 for a fine on a motor way. 

Thanks for reading

Advised by whom?

I should have mentioned that I received a letter of notice of intended prosecution from the police halfway down with this wording

"The alleged offence carries a maximum fine of £1000 (£2,500 if committed on a motorway) and a driving licence
endorsement of 6 penalty points or disqualification at the discretion of the courts. "

Do you know what maximum means?

"The alleged offence carries a maximum fine of £1000 (£2,500 if committed on a motorway) and a driving licence
endorsement of 6 penalty points or disqualification at the discretion of the courts. "

The 'maximum' applies to all parts of that sentence.  I guess you could argue some ambiguity that the maximum applies to the £1000/£2500 fine only...

However, that excess is eligible for a course to avoid the points or a 3 points £100 fixed penalty...  (The maximum's only apply to court - although there would also be costs and surcharges if the matter went to court)

Just read (again) https://www.ftla.uk/speeding-and-other-criminal-offences/read-this-first-before-posting-your-case!/