assuming that you mean:

1-299
300-399
400-

then try:

=LOOKUP(A1,{0,300,400},{30,35,40})

if the weight is in cel A1 you could try
=IF(A1>300,IF(AND(A1>300,A1<400),35,40),30)
you will need to change it slightly as you have 2 values for 300 & 400
kilo's.  You might also want some validation to prevent the cell having a
value over 500 kilo's

This formula presumes that you have a cell named weight and that weight is
not 0, and that any weight over 400 kilos is £ 40.  You can make it more
elaborate. For a few more weights, you could do a table lookup.

=IF(weight<=300,30,IF(weight<=400,35,40))

First I think you have a couple of typos.  You can't have two rates for 300 or
400

Maybe you mean  

1-300
301-400
401-500

or do you mean?

1-299
300-399
400-500

Will there ever be fractions like  299.96 kg?

Will there ever be more than 500 kg?

If so, would the max be £40

=LOOKUP(A1,{1,301,401,501},{30,35,40,40})

Try the above............adjust ranges as needed.

Gord Dibben  MS Excel MVP