Put simply, unless the Solo time is countersigned by the instructor who supervised it then you can't count it.
This makes for some difficulties. It has never been the case that a UK instructor has routinely countersigned each and every log book entry. The tradition in the UK has been to maintain a separate and comprehensive record, held by the school, although this might not always be the case outside the UK.
AMC1 FCL.050 instruction time:
this doesn't actually state certification must be within the individuals logbook and entered at the time. The UK norm was for the CFI to certify the logbook as being a 'true record' whenever this was required. I still have my rubber stamp and ink pad for the purpose.