I'm looking to Upgrade My Laptop Harddrive

If it's a 7778 or 7779 model, which is likely -- it is limited to a 7 mm drive or thinner.

Bear in mind that replacing a hybrid drive with a 2T conventional drive will slow the system down.

You can install any 2.5" hard drive as long as the height is supported. I see there are 4GB SSD laptop hard drives out there, expensive but it should fit. You can also install regular laptop HDD if desired. Check if your laptop has two hard drive bays or M.2 slot for SSD.

Inspiron 17 7000 series

You can use any 2.5" drive that will physically fit the system - depending on the model, you'll face either a 7 or 9.5 mm height limitation.  There won't be any drives at 4T that will physically fit the system.

There are some 2T 9.5 mm and even 7 mm  drives - you'll need to provide the full model number (i.e., Inspiron 7779) for someone to tell you whether you can use a 7 or a 9.5 mm drive.  Either way, if you really need that space, an external drive is much better idea.

I'm in the UK.. Won't this work, www.ebay.co.uk/.../112090391736

I hadn't thought about 4T as an SSD -- that is indeed an option, though it'll cost about twice what the system did originally.

www.amazon.com/.../B01G844OOO

My Inspiron 17 7779 came with a 7.0 mm or close to that.  I replaced it with a Samsung 850 Pro which fit fine.

I can not say for sure it would take one larger than that.

Be careful taking the bottom off.  there is a tab in the center near the back which is not mentioned in the manual.  you might be able to apply pressure in the center to get it to release backward but can't say for sure.

They can on SOME of the 17" models - they're not all equipped with M.2 slots, though.  That's why it's not possible to be definitive without knowing the exact model number.  Beyond that, you won't find any 4T drives in M.2 format.  

I think 2T is as large as an M.2 drive gets at this point - and it'll be well over $1,200 for one.

No, it won't.  That drive is 15 mm in height -- your system will take only 7 mm or slimmer drives.

www.seagate.com/.../