The options you have for your existing pension plan(s) will vary depending on the type of pension product.
Typically, you can keep them going, although if a pension plan is set up through a previous employer, you may not be able to continue making payments into the plan. If this is the case, the plan is made "paid up" - this means it remains invested and will incur charges as before, although you are not continuing to make payments into it. This could mean that your plan value will reduce by the time you retire.
Alternatively, you can transfer your previous pension(s) into your present employer's pension plan, although there may be restrictions on when you can do this and on the minimum amount of any such transfer and you may incur a charge.There is no guarantee that you will be better off.
When considering what to do with your existing pension arrangements, it is important to seek advice from a financial adviser. There may be a cost associated with obtaining advice.