Thanks for reply dude! And i plan to use TV, Playstation, laptop, phone charging?
Fridge or Coolbox as well?
Best thing to do is make a list of the kit you want to power like above, check the power consumption of each (this is usually written near the power sockets or on the transformers) and how long you are likely to run them each day and you will end up with a total daily power use.
So for example:
TV: 60W (= 5Ah @ 12V) x 4 hours = 20Ah
Playstation: 84W (= 7Ah @ 12V) x 2 Hours = 14Ah
12V Fridge: ave 1.2Ah x 24 Hours = 29Ah
Device Charging: 0.5A x 6 Hours = 3Ah
Laptop: 36W (= 3A @ 12V) x 3 Hours = 9Ah
Lights (LED): 1A x 4 Hours = 4Ah
(All numbers just assumptions

)
So in a full day in numbers above, you will use 69Ah. Shouldn't run a battery down more then 50%, so this would "use up" a 140Ah battery in a day. Obviously on hook up the power would come via the charger, and you will be topping up the battery at the same time, plus when you are driving, the split-charge will be recharging the battery, so the 69Ah draw will usually work out a fair bit less.
If you will usually be on hookup and just the occasional night away, a 100 or 100Ah battery would be good I would think. You could also in the future add Solar, which is pretty effective even in the the UK in the main camping season, if you need longer running off-grid T: