You only need to disconnect one terminal - black (-ve) preferred for safety reasons - if you can charge in situ. If you must remove the battery then just disconnect it - again preferably black first for safety, and when returning you can connect it up in reverse of the above.
Almost any smart charger will do, just remember that if the the battery is down, with a 6A maximum charge it will take longer to reach peak charge.
The Halford's charger will do, but if there is no rush just wait until the New Year when either Aldi or Lidl will have a similar rated device on sale for about £14. Many people on here swear by C-Tek but they are rather more expensive. I have had two smart chargers for years, one each from Lidl and Aldi, and neither has ever given a second's problem. The only issue with these two - and many more expensive ones as well - is that if the mains goes off the charger will not self-restart when the mains is restored, it needs human intervention.