I’ve seen a bunch of videos (eg on YouTube) which show a custard and chiffon cake recipe. This custard is essentially:  Egg yolks Condensed milk Milk or evaporated milk  My preferred custard is quite thick and uses the Delia Smith recipe, which I would usually put on/in a meringue (great with pavlova.) This is typically:  Cream Egg yolks Corn starch  I would like to use this custard recipe inside (or under) the cake, but I would like to know if it will split? Would baking custard in a cake be better using the condensed milk version or could the Delia recipe work just as well? I’m asking because I can’t try it now or any time soon due to travelling :( 
These aren't the same kind of custard. Delia's version is a traditional English pouring custard.  The other is a set custard. If you try to bake Delia's it will likely split and not set. I would use the recipe that is intended for baking. I'm pretty sure Delia will have other recipes that use a set custard though, as these are also pretty common here in the UK. In fact she does have one she uses for traditional English custard tarts 