1) Can I purchase hosting later?
2) What does 'UK Whois Opt-out' mean?
Sorry, can't help you on that one
My wild guess would be a method to hide your registration details from the UK WHOIS search engine. As with most domains, you're name, address and phone number will be listed (assuming you put correct info).
3) What does 'Setup Email Catch-all' mean?
Basically it will catch any email that is being sent to a non-existent address on your domain. So if you only had email@example.com it would receive mail as normal. If someone emailed firstname.lastname@example.org (and the account did not exist), it would be routed to the catch all address.