There are some basic principles, and some exceptions -
The basic departure point is the employment is deemed continuous in terms of length of service, and that the employment terms with the new employer may not be less favourable than with the old employer for 12 months, although certain changes are allowed sooner under certain conditions (e.g. you might be moved from one pension fund to another provided certain conditions are met).

At first glance it might seem the simplest way to ensure the new conditions are not less favourable than the old is to continue with the same employment conditions exactly as before. However, there may be circumstances where this is not a simple matter at all - especially if there is a significant difference between the conditions of employment for the same post between existing staff of the buying company and the incoming staff from the purchased company. (And remember - who's got the better existing deal might cut either way).