Forget planes...I think the point being made is that ISIS can afford plane tickets and false documents - if they want to enter Britain - they don't need to walk there with everyone else.
Do you know the number of boats/yachts there are in the south of England alone?
Of course you don't - and neither does the 'authorities'!
(estimates are anywhere between 100,000 and 1.5 million!)
Do you know how many permanent customs officers there are between Dover and Dorset?
Less than 20!
But hey, we can always rely on 'dad's army'!
A plan for the east coast: https://ukdefencejournal.org.uk/border-force-may-use-volunteers-to-help-guard-britains-borders/
Others here seem to not understand the logistics of patrolling the fourth largest coastline in Europe!