Not sure what you mean by keep no record as you need to state...

Not sure what you mean by keep no record as you need to state what the capital gain / loss is which is calculated by whatever you realised on exit less whatever it cost on entry, so you need to keep track of that much at least. I suppose you may not keep the contract notes to be able to provide evidence of the entry or exit but any online broker will store them for you anyway so should have a record. It is certainly the case that you can choose which parcel is being sold. I would imagine that in an audit the same amount of proof would be needed independent of which parcel you sold.