Whether or not the OP made the alteration there is still the offence of using the altered ticket for travel.
Most TOCs have a policy of prosecuting deliberate fare evasion and this is very much adhered to in cases of fraudulent activity such as altering tickets to make them appear valid when they are not, or making fake tickets etc.
I think it most important to give a realistic summary of what might happen
In my honest opinion the best the OP can hope for is that the TOC prosecution manager is one who will permit a settlement if there is no past history of any other ticketing offence. They may take into consideration some other factors, which I'm not going to elaborate on here, but make no mistake, this is likely to go to prosecution of the offence of:
'Having not previously paid the fare due did attempt to use an altered rail ticket with intent to avoid payment of the correct fare contrary to Section 5(3)(a) of the Regulation of Railways Act (1899).'
If convicted for a first time offence this carries a maximum penalty of a Fine of up to £1000.
On conviction the Magistrates will impose a fine from that guideline figure and commensurate with their interpretation of any aggravating features that the prosecutor may draw to their attention.
The TOC prosecutor will ask the Court to award their costs incurred in pursuing the case and to award the compensation of loss of fare for the journey made by the OP. The total of this might be a few hundred pounds in addition to the fine.
If convicted and fined, the OP will also be ordered to pay the Courts 'Victim Surcharge', which amounts to a further 10% of the fine imposed.
All of the fine and victim surcharge goes to Courts central funds, the TOC only receive any prosecution costs awarded and compensation of the unpaid fare,
How high the fine and costs might be will depend on how the OP pleads. If the OP pleads 'Guilty' immediately they receive the Summons then the fine and costs will be at the lower end of the scale as the Magistrates will take into account an early plea, but if the OP enters a 'Not Guilty' plea, a full trial becomes necessary. If convicted at trial the fine and costs imposed will be considerably higher.
If the OP receives a Summons and intends to plead not guilty the help of a good Solicitor who specialises in criminal law will be costly, but advantageous.
Conviction for this offence also results in a 'criminal record', an entry on DBS
If I were the OP I would prepare for this eventuality, but it wouldn't stop me from attempting to elicit an alternative disposal by being very apologetic and asking if the TOC will allow payment of all costs & compensation immediately in order to settle the matter without Court action.
One or two TOC prosecution departments are more likely to accept this than others.