I'll try to add to what's been said...
1. Do i need a licence,qualifications to own and use a boat (in the sea)
Legally speaking, no, but without guidance and experience it would be foolish to use a boat in the sea, so attending an RYA approved training course would be a very good idea. Powerboat level 2 (which usually includes powerboat level 1) would be a wise choice.
2. I have a budget of approx £1500 and want a 3m> rib am i living in a dream world.
Possibly... not sure if you're really after a RIB or a SIB? New SIB £600-£1100, New 3m RIB £900-£1500 and new 10hp engine about £800-1000. Secondhand obviously less, but beware of old unreliable equipment.
3. do you need a vhf (legally)
No, but if you venture into any tidal waters (including tidal rivers and estuaries, consider one, along with other safety kit like lifejacket. You will get good guidance on an RYA course.
4. what would be the maximum engine i could attach to a rib
Depends on what RIB, really. I would suggest about 10hp on a 3m. Manufacturers rate every craft, so this can vary from one manufacturer to another.
5. in a 3m rib is it still possible to have a steering wheel,electric start, throttle etc..
Yes, but don't bother on that size as there's not really any point.
6. Can a 3m rib be deflated if need be (for space reasons or if i did'nt use it for a few months)
Depends if it's a RIB (rigid hull) or a SIB (soft hull). With a RIB the outer tubes only deflate, making it slightly smaller. With a SIB it all deflates and folds/rolls up.
7. what would be the best make of rib for me (not bothered about pulling things but might like to dive off one and if i did would it hold 3 adults 2 of which would be diving or am i back in my dreamworld again)
Don't even think about "the best make" until you've a clearer idea about what you want.
At this point I would suggest doing the following:
1. Enrol on an RYA PowerboatL2 course. From Manchester I would suggest travelling to either Blackpool - Dave Mallett's Wavelength Training http://www.wavelengthtraining.co.uk/
or to Pwllhelli - Jono Garton's Professional Powerboat Training http://www.professional-powerboat-training.co.uk/
2. Go to RIBex at Cowes in June. Lots of RIBs and people who are interested in them.
Bear in mind that a 3m boat will limit your area of operation, but you can still have great fun. Keith Hart has a 3.4m SIB with an inflatable keel and floor (&15hp outboard) and has great adventures. I used to have a 3.6m SIB with an inflatable keel and solid wood floor (&18hp outboard) which was also good fun. At this size, the best compromise is the inflatable floor & keel type like Keith's IMHO.