The 700s were ordered by the DfT to a spec where the trains are not brought out right when completed. The owners, Cross London Trains or XLT, only get paid when a train has completed a days work. For full payment the unit must not break down, toilets, PIS etc must all be fully working with deductions made if they fail any of the setting. The same is true if Siemens does not have enough 12 cars and puts a 8 car out instead. The 20 or so banks are funding the fleet in the interim and expect to be paid as the units earn. However any alterations to the fleet requires all parties, ie Siemens, DfT and all those banks to agree with their own lawyers etc etc.
The 707/717 are ordered by a traditional ROSCO for a TOC and brought once they complete commissioning. Any alterations will be a matter for the ROSCO and TOC to sort with possibility the DfT stamping the deal.
As you can see the 700s are a lot more complex than the 707s to deal with. This is why the 700 spec was frozen in 2008 even tho the deal wasn't signed until many years later. The 707 deal is much more flexible.