Best Value "Safe" P2P Investment?

I would suggest AC QAA/30DAA offering 3.75%/4.25% as a good option, particularly as QAA has a fair degree of FSCS protection potentially.

www.assetzcapital.co.uk/blog/show/86/2016/05/19/fscs-protection

I don't interpret the blog that way.  Protection is available in two ways - for that (small part) of the QAA which is uninvested-I can't find any figures but my guess is well below 10% of your money.
The second way applies to any platform, if you can persuade a proper IFA to advise you to invest in said platform.

I favour Wellesley, bridging loans with reasonable ltv, automatically very well diversified, and especially because as well as a protection fund they take first loss so their interests are aligned with ours.  I wrote the words below in another thread 18 months ago.  I admit they read like an advert

lets say you put in 100
they will spread it around in sort of £10 lumps into 10ish of their current laons (which are 25k to 2M). Legally the borrower owes you so if wellesley goes bust your money is not lost
wellesley also put some of their own money into each loan (10% I think?), so if it isnt repaid or (more likely) is only partly repaid they lose money, so they care. In fact if there is a shortfall their money goes first. In other words lets say they had to foreclose on a 100K bridging loan (on a property which would be valued at 150K or more ie 65% max LTV)
if as is most likely the property sold for 140K, fine wellesley and you get paid in full and the borrower gets some of their own money back
If it sold for 90K, the borrower gets nowt, wellesley gets nowt, you get all your money (all your £10)
If it sold for 75K the borrower gets nowt, wellesley gets nowt, and you get 75/90ths ie you would lose £1-66 (note thats if BOTH the bridging loan wasnt paid off and if the property sold for half of what the surveyor said it was worth).
But no even then you would get all your money back because they have a provisions fund.

You would only start losing money if lots of loans went wrong and property prices in general went down by >40%.

as much as FS has a negative at the moment with the pawn loans you can still sell them pretty easy with a month or two months to go they are still highly popular and even if you do hold until the end FS has not lost money on too many of them 

wellesley also put some of their own money into each loan (10% I think?), so if it isnt repaid or (more likely) is only partly repaid they lose money, so they care. In fact if there is a shortfall their money goes first.

shimself , I may be wrong / out of date but when I delved into this a couple of years ago IIRC W’s skin-in-the-game model foresaw a fairly severe dilution of their stakes in their loans &/or recycling of their stakes out of the loan over its term. Meaning they may well have v little / no skin in the game by the time a loan gets into trouble. As their loan book grows (it grew very fast from the word go) their funding needs just to put cash into new deals becomes a steep hill to climb (could be a reason for their embarking on the bond route).

This, added to the over-slick marketing, pushing-the-envelope bond-raising & other things led me to conclude they were significantly higher risk than would appear, particularly to the casual investors their model & marketing is designed to attract.

A quick look at their website features the “we commit our own money alongside yours” very prominently, but I see no mention of how much that is, or if it’s all held to term. If someone were to pry this out of W I’d be interested to hear the result.

Smooth operators, & very successful to date. But perhaps a bit too smooth for my tastes.