WEDNESDAY evenings look set to become something of a battleground for TV programmes about budget travel. The Big Trip is a seven-week series on BBC 2 which begins this coming Wednesday at 6.50pm; one week later, Lonely Planet (based on the guidebooks) starts on Channel 4 at 8.30pm. Both series advertised for presenters, and each features newcomers to television. The BBC's selection procedure involved whittling 7,500 applicants down to seven, using a battery of auditions and psychological tests. Perhaps similar procedures could usefully be conducted on the population at large before letting us out of the country.