Can anyone direct me to where I can buy an off the rack one piece front fastening wet suit?

You may find this discussion useful: https://ukcaving.com/board/index.php?topic=26358.0

Personally, I am a fan of anything 2nd hand and cheap, it's only going to get destroyed caving! 

Look at the original post
Front fastening is the criteria.

Warmbac do one.

https://starlessriver.com/shop/warmbac_cavers_wetsuit

Here are three I looked at recently - but they all have attached hoods. With two of them at least the price is right, so you might be able to tolerate a hood even if you don't want one.

If you get any of these please write a review here after you've used it for a while - I'd be interested to know what it's like.

https://www.amazon.co.uk/Cressi-Monopiece-Wetsuit-Premium-Neoprene/dp/B00D2OYCC8/ref=sr_1_16?dchild=1&keywords=wetsuit+front+zip&qid=1590350918&sr=8-16

https://www.decathlon.co.uk/p/men-s-scuba-diving-5-5-mm-neoprene-full-wetsuit-scd-100/_/R-p-123153

https://www.lightinthebox.com/en/p/yon-sub-men-s-full-wetsuit-5mm-cr-neoprene-diving-suit-long-sleeve-front-zip-solid-colored-autumn-fall-spring-summer-micro-elastic_p7264047.html?currency=GBP&litb_from=paid_adwords_shopping&sku=1_45%7C2_291&country_code=gb&utm_source=google_shopping&utm_medium=cpc&adword_mt=&adword_ct=374723918144&adword_kw=&adword_pos=&adword_pl=&adword_net=u&adword_tar=&adw_src_id=3516490533_2081959428_74087838142_pla-528896766660&gclid=EAIaIQobChMI2vL505_N6QIVR0XgCh0OFgkeEAEYASACEgLdtvD_BwE

Tribal Chestnut said:

Warmbac do one.

https://starlessriver.com/shop/warmbac_cavers_wetsuit

Click to expand...

Warmbac do several. 

http://www.warmbac.com/listing_wetsuits.php

Pitlamp said:

If you get any of these please write a review here after you've used it for a while - I'd be interested to know what it's like.

https://www.decathlon.co.uk/p/men-s-scuba-diving-5-5-mm-neoprene-full-wetsuit-scd-100/_/R-p-123153

Click to expand...

As I have posted elsewhere, I have got one of these.  Actually, I'm now on my second since the first one got shredded beyond repair from digging trips to Wookey 24.  Its cheap, warm and comfortable though the neoprene is soft and not resistant to being ripped by rough limestone.  I should have worn an oversuit to protect it and perhaps done a few running repairs rather than just letting it get worse.  The fact that it was cheap (essentially free as I bought it with gift vouchers I had earned) probably contributed to its lack of maintenance.  Several other cave divers also have this suit.  I also have the 7.5 mm version which is good for longer cave dives. Having had several Warmbac caving wetsuits (both Cavers' one and the SpeleoPro0 the  these last longer by turn into cardboard.  They also cost more and leak through the seams.

TLDR: Its a wetsuit - it keeps you warm when it is wet.  Its very comfortable but the neoprene isn't up for gnarly caving.  Wear an oversuit to protect it, repair it regularly or treat it as a consumable (mine was "free").

Thank Duncan - yes, it was partly on the strength of your comments (which I greatly respect) and those of pwhole (which proved invaluable) that I opted for a Decathlon suit after that other discussion. I think it's really good and I'm grateful for your input.

I've still only worn mine once in anger since I bought the damn thing. Still, I'm sure it'll come in handy when we get into the 'new streamway'

Canyoneering wetsuits are worth considering, they have protection in most the right places!

Warmbac it is.
Thanks for the help folks.

Try it on before buying - they can be quite stiff. Also they have an inflexible seam at wrist and ankle, which makes it awkward to get over your foot. In fact the seams have several lines of holes through the stitching, which leak cold water. Warmbac suits have very good longevity but they're not the most comfortable I've worn.