The CPS and court won't give a flying f*** who modified it, your gun, your record...
If you suspect it's been tweaked either get it chrono'd and get it legal or don't get lifted with it!
HW77 is really nice, right up near legal out of box.
My question would be for that money why not get a decent secondhand PCP. Shop around, try airgunforum, if it's a hunting tool you'll want toys, bipod, decent silencer and scope etc. which there's a good chance of getting included.
Spring guns are nothing like proper guns, all the accuracy is in how you hold them, very dissimilar to traditional marksmanship skills. PCPs shoot just like a proper gun just recoilless.
Regarding .22 vs .177, I've got a .177 bsa ultra multi, it's alright. I wish I'd bought a .22 TBH, more for the extra shot count than terminal performance. .177 springers can kick a bit more too for the same physical reason. Old knackered BSA underlevers are easy to be accurate with when they have no kick... I've got a right old rolling block loading one myself
