225 hours is the RSSB Minimum. It also includes 40hrs minimum dark driving too. Again, this is directly from the RSSB and is a specified Minimum.
All the rules, regulations, guides, standards, partridges and indeed pear trees can be found on their website. Anybody wanting to play with 'fast' moving aluminum sardine cans needs to go through very specific training and meet various criteria.
You cannot become a mainline driver from a heritage route, 500hrs on TSW2020, or because you have a Hornby R186 Signal box in your attic.
Will it change...
You can take the assessments privately and DB Shenker run them from time to time. (not sure if this includes the MMI)
DMI (Driver Manager Interview) is your job interview for TOC/FOC employment because they are the ones who provide your training. FOC's I am unsure of as I don't know if they take trainees and run rules courses etc but I seem to remember that a FOC recently recruited for Trainees so maybe that is changing. High Speed (HS1) only takes fully qualified Drivers and are employed by Southeastern.
PTS/Rules... Might be changing in the long term future. It is slowly leaning towards becoming independent of the TOCs and there may be a possibility that you could pay for your own training. This is a little more than pure speculation but its still a way off and in railway terms that is a looooong way off.
I don't think there will ever be a way to get the practical side without being employed by a TOC/FOC. There is a lot involved behind the scenes and you need a lot of resources and money to train a Driver. If you had millions and millions of pounds I suppose you could start an open access operator, bid on a few paths and use it as a mainline training school. You would either go out of business in a week or make an absolute fortune.
It's also worth noting that you can't suddenly become a Heritage Driver either; even if you are a fully qualified Mainline Driver. They also have various rules and competency hoops you need to jump through too.
If you worked on a heritage railway, I certainly believe you would have the required skillset, mentality and experience that would give you a good foundation at each stage.
Each part, at some point, has been discussed on this forum. The magnifying glass isn't a zoom function and I would recommend those wanting further information to use it.