Hello - welcome to the Wonderful World of Piggies 
Do you want 2 pigs or 3 ?
If you want 3 then you're best with 3 sows or a 2 sow+neutered boar group.
3 boars are tough to manage even brothers or Dad+2 sons.
That hutch you've linked-
The upper level is what you need to measure as the living area. They'll go onto the lower level to pooter about but they do prefer grass.
Some guineas don't manage ramps well , you can make them more like a tunnel, put grippy bits on and make sure they're shallow.
You need to allow for the space that the ramp door takes out too.
I've always preferred pigs on one level
The area they'll use most s the wooden door area sleeping bit. How much space is there.
You'd need to secure it , foxes are the most (prepare for a 70 rant here) vicious, opportunist, stealthy, vermin who will have no scruples about taking any small rodents, killing them all because they can.
I've had one of my buried guinea pigs dug up twice , luckily he was sealed in a thick plastic bag, but I was gobsmacked how audacious foxes are .
I'm in Essex (so same sort of weather as you) , my guineas live in hutches inside a wooden playhouse. The hutches (5' long) are open onto the floorspace to allow excercise and they are put out into rabbit runs. I don't give them unlimited access because if I'm at work I don't think I'd be happy leaving them out.
If someone is in the house I get them to check every so often.
Mine come in for Guy Fawkes (noise) then about December they come in overnight until maybe late Feb.
I woudn't want my pigs out in the cold- they will snuggle up in hay and SnugglePads but its not good for them to be static.
Look at C&C cages for indoors (grids) you can make cages to fit an area.
As a rough guide 2 adults need 2'x4' minimum, I have a 4'x2' cage + C&C grids , my pigs are ok in the cage but they wouldn't like to be in there 24/7.
Early this year I had 2 sow+1 boar and 2 sows (so 2 cages/hutches)
Now I have the boar on his own and the 2 sows beside him (my guinea pig doesn't like him ) so I'll still need 2 cages for winter.
There are lots of adults in Rescue and some piglets if you'd prefer babies.
DD loves boars they are complete soppy Love Sponges 
, they don't smell (the hay is the only smell bit) but you need to keep them clean.
And boars need more space to prevent issues with territory.
Check any allergy potential - the hay and the guineas (my DD developed allergies with GP3 but won't let me have her current very allergenic boy)
Boars are IME tidier than sows. Our sows are feisty sassy bossy girls (but all my pigs are Rescued so I've always gone for Pigs With A Story ).
Insurance - I got a quote once but TBH you might as well save some money for them.
Most pigs don't need a vet but find a CavySavvy vet.
As a rough guide - we paid £58 to have GP3 neutered .
When mine have needed antibiotics it was ~ £30 for Appointment+meds
£35 to have GP5 put to sleep.
They need checked at least 2 x day, morning hello and breakfast.
Night time supper and lock up.
Cuddle in the evening check for any injury, lumps bumps.
They get tame with handling and are very noisy
I do morning duty, DD puts them in the runs when she gets home from school, we do evenings together.
You'll need to think what you'll do when youre away ( Boarding is £5 ish per pig/per day)
They are my favourite little animal. We're on GP6/GP7/GP8 at the moment , all 3yo now.
Haven't been forgiven yet for cutting GP8 claws and bathing GP6 and cleaning his little ears !