Unless working with you will greatly benefit them (your reputation will be a huge boon to their own reputation, or they have a project you would be perfect in), they won't do it for free if they're any good. Skilled photographers have worked for free so many times, they're tired of it. New photographers.... well, they won't usually get great shots, but they're available and in need of experience. There are a lot of contact sites for models and photographers in the UK. Just give it a google.
Most won't be willing to do it for free unless they are starting out, just like you. Sometimes, in certain circumstances, a photographer may have a particular shoot that they want to do for their portfolio and you fit that concept!
Photography is expensive, time consuming, and a business. That's just the fact of it. I know I won't work for free unless it very significantly benefits my portfolio, I just can't afford it unfortunately, no matter how much I love what I do.
ModelMayhem is a pretty good bet, you may find someone willing to to a TF (trade for _insert here_).
True, which is why I think it is good that he go to a site like model mayhem and find photographers who have his level of experience. That way BOTH of them get something out of the shoot, it just isn't money which is pretty much my policy for how I run things myself. If I am working with a model of about the same level experience as I am a photographer and we are both looking to create something for our respective portfolios then the mutual benefit is absolutely worthwhile to work for free but if an entry level model approaches me wanting me to do a shoot "for" them they must pay since I likely will get little or no benefit out of the shoot otherwise.
Perhaps, but honestly, i'd argue that you stand to learn just as much from an inexperienced model. Working with someone that is far superior to your skill level means that you may be able to avoid problems you otherwise would have to overcome.
Furthermore, TFP is just how much of the photography industry works these days, many of the top photographers in the world make a point of continually doing free shoots to sharpen their skills. Lara Jade claims that as much as 70% of her shoots are personal assignments and involve unpaid talent. Joel Grimes makes a point of shooting at least one TFP shoot per week and never puts actual client work in his portfolio. Paying models for upwards of 60 shoots per year simply doesn't make sense when you have access to ones willing collaborate for mutual benefit.
Personally i'd say working with a free model and putting that money into a workshop with an experienced photographer likely will yield more learning opportunities than had you hired an expensive model.