Driver training is probably a big headache / limiting factor too not just the stock...Latest report is that from first unit into service (notably, no date given for that!) it'll be 2 years for full fleet replacement, so less than 1 a week on average
DfT may also have been involved and having some of the new LNR units not being delayed even further might be part of things. e.g. LNR not following on after Anglia /SWR but in parallel later on.