And as for Customs - UK AIP GEN Section has the fax numbers of the Customs offices who you have to notify, depending on your area of arrival. Normally 4 hours notice is required of an arrival from within the EU.
As to the flight plans - use the interactive Flight Plan form at
www.ais.org.uk to fax it to the relevant FPRS
Any other queries feel free to drop me a line direct
CM