I've interviewed and taken on staff in the UK, (occasionally) in my past 20 or 30 years. Some thoughts / comments; sorry if they're a bit late but we've been away-ish for a week.
First, if there's one vacancy available, an employer will fill it with the best possible person for the role. You might feel you're ideal for a job (and you might be) but if there's someone who's "more ideal" you'll loose out to them. You might feel discriminated against, because friends / contacts have fallen into similar jobs they've applied for with lesser qualifications; indeed, it might be discrimination, but it could simply be that in your particular race there tougher competition. Don't loose heart.
But, yes, there ARE potential employers out there who may discriminate against non-UK applicants. Apart from anything else, the UK government makes it the responsibility of each employer to check that new employees can legal work, and threatens severe sanctions if they take on someone they shouldn't; this frightens off a number of potential employers. Whether this is "racial discrimintaion" I wouldn't know - I'm not a lawyer, but it's certainly not based on skin colour or religion. Race and nationallity are slightly different. me thinks. But if an employer can't see your worth, do you want to work for him anyway?
Other questions that come up in the employer's mind "is this person looking for a permanent job, or just passing through the area" and "is this likely to be a sue-happy employee pushing his / her rights to the extreme" and if the potential employee is customer facing "Are there likely to be misunderstandings because he / she speaks a different varient of English". Finally, there's the question "if this doesn't work out, will I get blamed for hiring the wrong person" - a great one for middle managers who will go for the safe option of the local lad or lass who may be slightly less qualified, but whom they won't get critised for taking on if it doesn't work out.
There's also been talk of accents and receptionist jobs here. The person who answers the phone is the first ambassador for the company, and should be a bit special. Any qualities they have that can help establish that in their first few words to the customer are worth their weight in gold, and such qualities include a very clear but non-UK accent. In the right situation, it says to the caller "here is someone who's not just been hired out of the local school, but rather someone who knows a bit", and I've seen this work with French, Canadian, South African and American receptionists. But attitude and how's it's projected (at interview and later) is even more important.
Now putting all of this together, it actually means that it's harder for the not-run-of-the-mill candidates to get job offers, but when they do, they'll typically be with good employer and the interviewer will be especially wanting them to work out well.