I don't know what the availability is like in the UK, but you might want to look at the Enell bra. It isn't pretty and it's a pain in the butt, but it will hold your bosoms in place while you bounce. Apparently it's Oprah's preferred sports bra. I ordered mine from Ebay for less than I would pay retail. It was a godsend when I was doing aerobics. Now I use a rowing machine regularly, and I just use a regular bra.
Here, here on the not making workout clothes to fit normal sized women! I wanted to buy this cool underwear thing for riding your bike, and the largest size they make is a US10. You've got to be bleeping kidding me!
Depending on how fast you go on the rowing machine, you may still want to wear a sports bra. You'd be surprised how much movement there is involved in rowing and cycling and how you still need to wear a supportive bra. I wore my Enell for years rowing, but have switched to the PureLime as the Enell doesn't do small around/big boobed. There's no way I could row without a sports bra - and I've tried!
As for cycling gear, I have two sites for you:
www.wiggle.co.uk (lots of women's kit on a dedicated page)
www.alwaysriding.co.uk (again, women's kit on dedicated pages)
Both of these shops do an XL in cycling bottoms which would fit a size 18 with normal legs or a 16 with really thick legs. Additionally, REI in the USA ships to the UK and they do plus sizes in a lot of the cycling and workout gear.