Someone I know doing it says that the school is obliged to hold a reception place open to her son, and offer him another one next year, meaning that they will have an empty space with no funding for this year.
I think she must have misunderstood, as there is no obligation to do this.
A parent has the right to defer their child's start date until they reach statutory age, so once a place has been given, they can delay starting their child until January or Easter in that reception class if they wish, and that is their statutory right. The place in that class will be held open for them until they do so in that case.
For a summer born, this might mean 'deferring" until the following September, but in that case, they will normally go into Yr1, unless prior to their application, they agreed with the LEA that they can delay school entrance for a year and go into reception "late". (as discussed endlessly above!).
What they can't do is accept a place for reception this september, defer for 3 terms, and then say "actually we want a place in reception for 2017 , thanks". The decision to enter YrR this Sept but delay entry until Jan/Easter, or to request to defer entrance and enter YrR next Sept, is a decision that needs to be taken now. It's not a choice she gets to make at some point in yrR. If she accepts the place for this Sept, that is the school place she has, and it needs to be taken up at some point in this academic year, or in Sept as entry into Yr1.