I don't think you'll get a reply from the original poster as it's been 13 years. I think no one can give you a definitive answer as there are so many variables. I haven't met a doctor changing to vet, but there are people who are over 30 doing the vet degree from various backgrounds. From a technical standpoint, obviously you've considered the funding fees which are very expensive as a second degree. It is ridiculously competitive to get in, especially for postgrad courses, but I assume you'll be working during so it won't be as big of an issue if you had to reapply etc. I'm not sure entirely what the rules are for your case in terms of qualifications, I'd make a table with all the different unis you'd apply to and their individual requirements as I know it can get a bit complex - some requiring you to do access courses or A Levels even if you've had a first degree etc, and if there are any grey areas do email the unis to clarify and have anything in writing. You will also need to do a number of weeks of animal-related work experience in both clinical and non-clinical settings, I'm not sure how easy that will be for you to fit in between work. And I'll assume you've considered any personal challenges (kids/location with house/financials etc) which aren't impossible to do the degree with but might just make the journey that bit harder.
I wouldn't say you're mad, but I would consider your reasons why. Obviously there is no NHS for vets but right now a lot of places are understaffed as there isn't enough vets, so working conditions in terms of staffing may be similar. Your job is still very human-facing and owners can cause the majority of your daily struggles. With owners having to pay you are often very limited in what you can do, may have to PTS animals which could've had a chance with more money, you're always known as 'money grabbing' and 'if you really loved animals you'd do it for free' etc when your salary is much less than a lot of human doctors, with the average being 40k when you are qualified as a full surgeon. Unless you specialise it is also a very broad career as you are treating every body system, lots of different species, usually doing both consults and procedures unlike a lot of the human medical field, but from what I've heard from medical students (I may be wrong) you do also go into more detail with all these body systems on the vet course since you are expected to know how to treat them as opposed to specialise later. You'll have to do 36 weeks doing EMS placements which are usually unpaid (although I've heard it may be lowered to 30 in the next few years), and for preclinical may not always be the nicest as you can easily be working for 16 hours in the middle of the night lambing ewes in the freezing cold rather than just shadowing in a hospital. A lot of your patients will probably be scared of you and the owners can be a little mad. There is a lot of issues with poor mental health amongst the vet career with all these stressors.
Not trying to put you off and I'm sure you already know a lot of these and I'm not saying working as a doctor for the NHS is much better/easier, but these are things to consider on the off chance you are looking at a vet career with rose tinted glasses - the grass isn't necessarily greener. But if this is what you have always wanted and feel you are prepared to cope with all of it and find it a rewarding career, go for it! You've got many years left and if this is what you think you want to go into, as they say if you get a job you love you'll never work a day in your life. I think this decision is very individual and personal depending on your circumstances, personality, motivation etc. If you've got any questions I'm a second year vet student at Liverpool