An executor is entitled to claim out of pocket expenses from an estate. As to whether they are valid expenses are a different matter. Your first port of call is the Will. If the testator has asked for a knees up and the expenses to be borne by his estate, that is fine. The term "out of pocket" expenses has no particular definition - the above would qualify but probably best to ask the executor for a breakdown (which you can do if you are a beneficiary).
HMRCS allows some expenses -
http://www.hmrc.gov.uk/manuals/ihtmanual/IHTM1 0375.htm
At law the testator should be buried in a manner suitable to his station in life (Ie one would not expect an unknown pauper to have a massive funeral service at St Pauls). Strictly speaking I believe that flowers are not a mourning expense (but it is probably not worth objecting to). A wake is allowable, but it has to be reasonable. However, non of this is absolutely clear cut. The cases I have quickly looked at allow some things and not others, but it is fact sensitive.
However, if the estate is insolvent, all that is allowed is the cost of the internment and coffin.