Any rides for beginners?

Firstly, thanks everyone so much for the advice. Love this forum!

Richmond Park is always a good place to discover your capabilities. Ride out there or get the train and then ride the 7 mile loops in the knowledge that there are cafes to stop at and feed or rest should you need them, the hills are slight and short and there are plenty o other cyclists around. You will find that you can ride far further than you think you can. Once you have done that, you will have an idea of the distances that you can manage and then look for more varied routes. You will also see how you compare with other cyclists and may realise that you are ready for a group ride sooner than you imagine.

Yeah I guess I will give Richmond Park a go, it's just that getting there is a bit of cross town mission, and I'm sort of biased against doing laps of parks because I do that a fair bit at Regent's Park and get bored just going in a circle over and over again. Although I hadn't realised it was as long as 7 miles for one circuit, which makes it much more appealing. Will give this a try. Yeah the idea is to work up to a point where I'm confident enough in my own ability to happily join a group ride.

^ Your immediate problem is that to get to almost anywhere where you have the opportunity of both quiet roads and pretty views (I take that as countryside) will mean riding more than 12 miles from central London anyway so you are already at your 25 mile round trip limit before you start.
Looking at London on Google Maps Satellite view, the Lee Valley looks a good bet as it's around 6 miles from Tower Bridge but I'm not familiar with that area so can't comment on it's suitability for cycling.
My other suggestion would be head straight out 25 miles or so and then just get the train back. This will massively increase the available opportunities for variety of rides and routes and give you many miles more coutryside to enjoy.

I had realised this but was hoping that there might be somewhere slightly quiet and slightly pretty within the M25 - not countryside but just somewhere pleasant to ride. Thanks for the tip about the Lee Valley, I will have a look... just seen the following on their website as well, which sounds fantastic:

Covering 10,000 acres Lee Valley Regional Park is a cyclist’s paradise. Explore riverside trails, scenic country parks, rich nature reserves and urban oases. There’s over 27 miles of pathway linked to Sustrans National Cycle Network Route 1

Also thanks for the idea about returning by train. If you were going to cycle somewhere about that distance away and return by train, where would you go?

Which area do you live in? There are various convenient destinations by train from where you can start. Check out the Tuesday Night Ride Club routes list:

http://www.lfgss.com/post1990472.html

You're rather spoilt for choice there. :) These are all slightly longer than what you want but there's a lot of variety--hilly, flat, moderately hilly.

I live in Maida Vale (W9). So probably North and West London are better for me, but I'm open to suggestions anywhere. I hadn't seen that list before, thanks for the heads up. No excuses now, gonna go for a long ride some time next week!