Borrow to Invest?

The strategy is workable and not just with p2p. For example I have borrowed money invested in stocks, shares, fixed rate bonds, retail bonds and p2p. It is the desired outcome of central bank policy to get money into the economy by pumping up liquidity and injecting money in the system. It is, after all what a large chunk of investment bankers do for a living but on a huge scale reaping huge rewards (for little work (or non productive) IMO). The problem is this is not a free market and money markets are distorted. The likely outcome is that one asset class eventually underperforms and these huge leveraged bets go bad (think US property 2008-9).

Its insane.

Great point, the reasoning about using Assetz and Growth street was 30 day access to at least £4000 (2k in each) but yes I could try and get it invested at 10%+ such as Moneything, Funding secure and so on but that would mean putting all the money away for the duration of the loan assuming it cant be sold on a secondary market.

I'm a big fan of stoozing/cashback and other ingenious ways of maximising returns for the less wealthy investor, but I have to say this sounds like a really bad idea. I'm pretty much just going to re-iterate the points others have made in bullet form:

1) Over-leveraging yourself is inherently very risky.
2) Defaults and bad debts will happen, and you won't know when. They're unlikely to be at convenient times.
3) Similarly, you are likely to find a reasonable proportion of your loans will repay, but will be locked in for months/years beforehand due to unforeseen problems.
4) As you are unlikely to have a full grasp of all the foibles of these platforms, you're likely to make a few mistakes which will not be forgiven by your loan provider.
5) You will really struggle to get your £10k invested (or any amount) with any significant diversification at the rates you're after on day 1.
6) Cash drag will impact your returns both at the start and throughout.
7) You are eating into your own, very uncertain, returns by adding the cost of a loan to proceedings. I'm sure there are people here who would confess to coming very close to making a loss, or close to it, due to unfortunate P2P results even without having this cost. I am on Zopa+ just about, for one!
8) You will feel pressured to keep the funds invested even when no decent loan opportunities are about because you're paying for them, making it more likely you'll make bad decisions.

TLDR - I implore you not to do this.

To be less of a negative nelly, why not investigate these ideas (not advice):

1) Save a little, then take advantage of the best P2P sign up bonuses one at a time as they pop up, providing you also like the platforms.
2) Open bank accounts, savings accounts, credit cards just for cashback from quidco/topcashback.
3) You could look at matched betting (just google it) - again, there is risk here so this is not advice from me.
4) Use said bank accounts for promotional/ongoing high interest offers e.g. Nationwide £2500 @ 5% for a year.

I am confident that you can make the equivalent of 6-7% interest return on £10k i.e. £600-£700 a year by trying some combination of the 4 above alone.

Sounds like your level of leverage to cash is fairly sensible. I have seen people do all kinds of crazy things like use credit cards to bet on derivatives markets. My personally view of investment has always been never play with more than you are willing to lose, for me personally slow steady growth wins in the long run. That said, everyone I know who is wealthy has at some point levered themselves upto the hilt to take a gamble, but at the same time I also know people who have lost everything and had their lives fall apart doing the same thing.

I have as part of my whole retirement plan (2 years ago at 52) extended my mortgage until age 80 on a repayment basis.  This is cheap money at current rates, if rates increase excessively then I can reverse the decision and pay off the mortgage rather than invest the money in a number of different ways. I also have a large amount of equity in the house if things go pear shaped.  

The new pension freedoms have allowed me to do this as I can access drawdown from age 55.  I even (many on here will say no way!) transferred a couple of final salary schemes into a SIPP that I manage myself - the advantage being that I can access at 55 not 65 and I can withdraw based on growth and need.

All managed in a huge spreadsheet, monthly until age 80 (sad) with all income/outgoings different scenarios , inflation factored in etc.

What it has definitely taught me is what poor value annuities are. 

Curious, why 80? Life expectancy is closer to 85 and not impossible to hit 100?