Flights:

Here are some of the airlines that fly from the UK to Hungary:

Hungarian Airlines:
www.malev.hu

Low cost airlines:

between London and Budapest:
www.easyjet.com
www.wizzair.com
www.skyeurope.com

between London and Sármellék (Balaton Airport)

Airports in Hungary:

Budapest International Airport: www.bud.hu

Fly Balaton Airport: www.flybalaton.com

There are three options to travel from the airports to Siofok:

By train from Budapest:

Airport transfer:
From Budapest to Budapest Railway stations:
http://www.minibus-taxi.hu/angol/index2.html
http://www.besthotelsbudapest.com/airport%20transfer.htm
http://www.bud.hu/english/transport
http://www.railbus.hu/

Train schedule:
www.elvira.hu

There is a train to Siófok almost every hour all year long.

By transfer from Budapest and Sármellék:

From Budapest-Siófok:

http://www.siofok-taxi.hu/repuloteri_transf_eng.html
http://www.siofok-airport-transfer.hu/index.php?lang=en

From Sármellék to Siófok:

http://www.siofok-airport-transfer.hu/index.php?lang=en
http://www.fly-car.hu/index.php?page=transzfer&lang=en

By renting a car:

From Sármellék please visit:
http://www.flybalaton.com/index.jsp?id=3&lang=en

From Budapest

http://rentacar-budapest.hu/frameset.html
http://www.autorent.hu/English/index.htm
http://bestcarrental.hu/english/

and all major worldwide car rental agencies:

www.hertz.com
www.avis.com