if i take my sky+ box to spain, catalonia, will i still be able to watch the recordings i have made on it without getting a spanish sky subscription

If you still have a valid UK card you can watch recordings. There is no such thing as a Spanish Sky subscription.

To watch and make recordings, you need a signal from the satellite, and use the Sky UK viewing card that is paired to that Sky+ box.
If you are moving to Spain, best keep your SKy UK subscription active, but move the account to a UK address that you can control (friend or family), as Sky UK will not move the account to a non UK address.
Sky do not sell their service to Spain. In Spain there is Movistar+, which is the SPanish satellite / pay TV service.

Will the Freesat box made by Humax - available from any Argos store - work just as well in Spain as Sky's equipment in terms of picking up freeview channels?

Or does Sky's hardware fare better in areas with a weak sygnal compared to the Humax range?

Yes a Freesat box will work in Spain
As long as you have the correct sized dish to receive the signals for where you are in Spain.

You will not get Freeview, which is UK TV via a TV aerial, in Spain.
But most of the channels on Freeview are available subscription free on satellite via Freesat or a Sky box with no card.

Have found little difference in Freesat and Sky HD boxes reception wise.

If you don't want to reply on a Satellite Dish the new NOW TV box is excellent as is the current Amazon Fire Box - I have both and there is a lot of content.

Thanks, regarding the Freesat box will there be any issues if I never connect it to wifi (since it will be used abroad anyway)?

It doesn't need to connect to the internet to activate or to work properly, does it?

It only needs to be connected to the internet (via a VPN or smart dns), for access to the on demand / catch up content.
LIve TV is direct from the satellite, and it will work fine without an internet connection for these