I'd like to know the real impact it has had in terms of poor credit score
I'm not in a DMP but deal with those who are regularly through work. The short answer is that your credit score will be in the bin for many years. But I agree with a pp about why do you need a good score anyway, it's not always a key consideration if you're drowning in debt.
Personally, if you're fairly financially savvy (debt situation excluded!) then I would stay away from a DMP and try and do it the DIY route.
DMPs in practice have the potential to be rife with problems. Essentially the DMP provider will just contact the companies you owe and offer £x per month instead of the full amount. It will be accepted, you start paying your credit card £20 month instead of £150 or whatever.
Then one of two things happen. Either the lender will close and default the account in the next 3-6 months as you're not meeting your contractual minimum payment (default will stay on your credit file for 6 years from the default date then the whole account disappears). OR the problem bit - some don't. Instead, they mark you as being in an Arrangement ro pay and the account remains live. The problem with this is that, say you then manage to pay the debt off over the next 4 years, then the account closes. That account (and the arrangement marker) will still be visible on your credit file for 6 years from the closure date. You've just dragged the impact out over 10 years, instead of the 6 years it would have been with a default.
There are other problems that sometimes occur with DMPs, too many to mention.
This is a controversial approach but essentially the same aim as a DMP but self managed and ime with a better outcome. The DIY DMP route is simply to stop paying your unsecured debts. No contact or payments, just stop and go awol on them. Watch the process. You'll get missed payment letters, reminders, calls, a default notice, then a final demand, then a default on your credit file and closure of the account. But with this approach you can be CERTAIN that the account has defaulted and closed, removing the chance of the 'arrangement marker' problem.
As soon as that happens, make contact. The remaining balance will be with the banks debt collection arm or an external company like Moorcroft, Lowell, Wescot etc. Make a repayment arrangement directly with them for as little as possible - the bar is low, acceptance of £5 or £10 a month, without ANY review of your circumstances, is common.
Do that for all your debts paying as little each month as they'll agree to and then save what you can like mad. And after 6 years, all Defaults will completely disappear from your credit file although the debt itself still exists - so you have to keep paying them until you reach a settlement.
What will happen though is that the closer you get to that 6 year timescale, the bigger the settlement offers will become - I've seen many examples of 80% discounts offered at year 5 for example, with the account then considered closed.
People will warn you about the risk of CCJs with a DIY route - but ime, CCJs are only pursued for very large single unsecured debts (£20k plus) where it's clear the debtor has significant income or wealth yet still wants to pay a fiver a month - or for those that are unengaged and don't arrange monthly repayments after default and ignore contact from the debt collection agency.