As ex-London URNU I can heartily recommend you joining. The medical REALLY is no biggie - as with the RN, be within 25% of the required BMI, don't tell them you have asthma, and don't be colour blind.
Comparision of London URNU vs London OTC:
OTC
-500 ish cadets under a Lt Col and all the extras that brings...
-Called 'Officer Cadet' -but treated like 'orrible little privates by your RSM
- deploy in a trench, eat your rat-packs cold to get the full infantry-soldier experience, then catch a quick 2hrs sleep at most (literally), get woken up by flashbangs, then fall back to your 2ndy harbour postion just in time for your stint on stag...
- Get recruited into the TA when you leave IF you're not going to RMAS - otherwise they feel as though they've wasted effort on you.
URNU
-50ish cadets under a generally chilled-out Lt RN
-Given substantive rank of Mid RNR (URNU) and therefore get to experience a taste of wardroom life (don't expect to
be called Sir/Ma'am though - you are not properly commissioned Officers!)
-Deploy on a nice little patrol boat with heating, showers and a galley (its the small things in life...) so you can have a hot curry with your oppos etc. before your NIGHTLY run-ashore - NB, you will spend more on runs ashore than you earn in an URNU Summer deployment!
- Lots of URNU join the RN (I did) but the actual idea is based around giving future movers and shakers a positive attitude towads the RN
I knew the options infront of me when I joined the URNU and I'm glad I made the right choice for me. Lots of people (lots of friends of mine) loved the OTC, even those who haven't joined the Army - I howver was content to bait them in the Common Room on Monday mornings after I'd spent the weekend on a clubbing cruise to Bournemouth and they'd hiked half of Hampshire, humping half their body-weight around to the sound of a screaming Welsh RSM!
I hope this helps you in your decision making!