I wanted to unblock my bath drain but as I started to unscrew the drain I realised it wouldn't come off and now the waste pipe and some sort or rubber washer are now slightly out of whack. I've screwed the drain back but worried if I have a shower the water won't go down the pipe. The bath is tiles in so can't remove panel. Have I made a serious error?

Can you post a picture of what your unscrewed? Chances are that this was just a drain screen if it had round holes with one scre in the center. To unblock the drain remove the over flow plate and rod down that line using a small 1/4" snake

I wanted to unblock my bath drain but as I started to unscrew the drain I realised it wouldn't come off and now the waste pipe and some sort or rubber washer are now slightly out of whack. I've screwed the drain back but worried if I have a shower the water wont go down the pipe. The bath is tiles in so can't remove panel. Have I made a serious error?

Bob's wondering what plate you removed. Was it the strainer on the drain or the trip lever tub waste,(see image)?
To snake the tub Unscrew and remove the overflow plate,(The one with the drain lever) and pull out the tub stopper. You now have a clear shot at the trap. If you were going in through the drain it ran you square into a tee that you can't get around. Now feed the cable( 1/4" best or 3/3" with a small tip. You can rent a Ridgid K50 sink cable.) About 2' down you will run into the bend of the trap. Crank and push at the same time to work around it. Your blockage will be found in the lateral pipe going to the stack. You shouldn't have to put out more then 10 or 12 feet. I have found tub and shower clogs to be mostly hair and the worm should auger into it and pull it back. Hope this helps and thank you for rating my reply. TOM